1 Introduction

There are several provisions in the Vienna Convention of the Law of Treaties (VCLT), reflecting customary law. Already in 1969, the VCLT was considered to be a partial codification of customary international law (CIL). Other rules of the VCLT constituted then developed into customary law.Footnote ¹ Indeed, according to the commentaries to the VCLT the large majority of its provisions currently reflects custom.Footnote ² As international investment law is a branch of international law based primarily on treatiesFootnote ³ it is not surprising that the provisions of the VCLT and corresponding customary rules have been often interpreted and applied by investment tribunals. Considering that the total number of investor-State dispute settlement (ISDS) cases had reached 1,190 by the end of 2021,Footnote ⁴ investment arbitrationFootnote ⁵ has the potential to significantly influence the interpretation of the law of treaties, both in the rules contained in the VCLT as well as those confirmed by CIL.

The situation of parallel existence between treaty and customary rules demonstrates that it is not only a treaty’s rules, but also the customary ones, that in practice can be, and in fact are, interpreted.Footnote ⁶ Furthermore, this scenario in particular proves that the content determination of customary rules can be, and is, accomplished through a different approach than the ascertainment of two classical elements of custom, that is, State practice and opinio juris.Footnote ⁷

This chapter will focus on two issues discussed broadly by investment tribunals: rules on conflicts of treaty norms (Article 30 of the VCLT and corresponding CIL) and rules relating to subsequent agreements in relation to the interpretation of treaties (Article 31(3)(a) of the VCLT and corresponding CIL). Emphasis on these two examples of the interpretation of customary law in investment arbitration seems to be particularly pertinent as, in these two areas, tribunal’s decisions seem to diverge from the approach, as reflected in the works of the International Law Commission (ILC), traditionally taken in general international law. Why do investment tribunals deviate from agreed understanding of rules on conflicts of norms? How they define the scope of these rules? And with respect to subsequent agreements, can they influence the CIL concerning the interpretation of treaties which envisage rights for individuals? These issues certainly call for a study on the matter.

The aim of this chapter is not to provide an extensive and exhaustive list of all such cases where the interpretation of the customary rules codified in Articles 30 and 31(3)(a) of the VCLT has occurred but rather to highlight some most interesting examples which demonstrate the general approach of investment tribunals to these norms.

2 Interpreting Rules on Conflicts of Treaty Norms in Investment Arbitration

The issue of resolving conflicts of norms originating from different legal acts is central to every legal order.Footnote ⁸ In international law it is regulated in Article 30 of the VCLT and corresponding CIL. From the perspective of international investment law, the main issue that has arisen in the interpretation of this legal norm relates to the material scope of the entirety of Article 30, ie the reference in the title and paragraph 1 of this provision to treaties ‘relating to the same subject matter’.Footnote ⁹

The jurisprudence of investment arbitral tribunals has mainly referred to Article 30 VCLT in cases concerning the relationship between intra-EU investment treaties, or the intra-EU application of the Energy Charter TreatyFootnote ¹⁰ in relation to the Treaty on the Functioning of the European UnionFootnote ¹¹ (TFEU, or its predecessor – the Treaty Establishing the European CommunityFootnote ¹²). In the former case, Article 30 of the VCLT was invoked by the respondent, most often a Central-Eastern European State, as an argument emphasising the priority of the EU Treaties – subsequent treaties, over investment treaties – earlier treaties, ie dating back to the 1990s. At times, arbitral tribunals have also commented on the concept of the ‘same subject matter’ against the background of the applicability of Article 59 VCLT, which is also considered to reflect a customary rule, and then referred their conclusions to the applicability of Article 30 of the VCLT, even if they noticed differences between the purposes of these provisions.Footnote ¹³

As the ILC indicated in its report on fragmentation, adopting a narrow interpretation of this formulation could result in a number of potential treaty conflicts not being covered at all by this provision.Footnote ¹⁴ The Commission emphasised that:

The ILC, therefore, opted for a flexible approach to the formulation of ‘same subject matter’. Support for this position can be found in the Commission’s commentary to the 1966 Draft Articles on the Law of Treaties,Footnote ¹⁶ where the view was expressed that this formulation was intended to broadly cover cases of incompatibility between treaty norms. This issue has rarely been a subject of consideration by international courts and tribunals. Similar reasoning to the position of the ILC can be found in GATT/WTO decisions. In EC – Imposition of Anti-Dumping Duties on Imports of Cotton Yarn from Brazil the panel stated that on the basis of analysis of a single of provision of the Multifibre Arrangement (MFA), the Anti-Dumping Agreement and the MFA were treaties ‘relating to the same subject-matter’.Footnote ¹⁷ In the China – Measures Related to the Exportation of Rare Earths, Tungsten, and Molybdenum the Appellate Body acknowledged that all multilateral trade agreements annexed to the Marrakesh Agreement relate to the same subject matter without detailed examination.Footnote ¹⁸

Arbitration decisions, however, have adopted a narrower understanding of Article 30 VCLT and, consequently, of the customary norm that this provision codifies. As was stated by the Eastern Sugar Tribunal, ‘[w]hile it is true that European Union law deals with intra-EU cross border investment, say between the Netherlands and the Czech Republic, as does the BIT, the two regulations do not cover the same precise subject-matter’.Footnote ¹⁹ The Tribunal underlined inter alia the existence of fair and equitable standard,Footnote ²⁰ as well as the possibility for an investor to sue the host-State directly, as grounds to reject the ‘equivalence argument’.Footnote ²¹ This approach has been upheld in many subsequent arbitration awards relating to intra-EU bilateral investment treaties (BITs).Footnote ²² In EURAM, the Tribunal rejected the interpretation that the ‘same subject matter’ can be equated to being applicable to ‘the same facts’, or having ‘the same goal’.Footnote ²³ The decision then subsequently elaborated that ‘[t]he subject matter of a treaty, in the Tribunal’s understanding, therefore differs both from the concrete situations in which it will be applicable and from its goal’.Footnote ²⁴ For this Tribunal, the crucial argument was that prior to the Lisbon Treaty the EU had no competence in relation to direct investment.Footnote ²⁵ A narrow interpretation was also presented by the Strabag Tribunal which ‘under[stood] the precondition of “same subject matter” as requiring the subject matters of the two treaties in question to be “identical”’.Footnote ²⁶ Again, a similar position was expressed by the Juvel Tribunal: ‘[t]wo different treaties may apply simultaneously to the same set of facts without them having the same subject-matter. Further, if two treaties have the same goal but approach the achievement of that goal from two different perspectives, the treaties do not have the same subject-matter’.Footnote ²⁷ The negative position of these tribunals, towards the applicability of the conflict of norms provisions, was primarily related to the far-reaching consequences that States could derive from the arguments in this regard, ie lack of jurisdiction of those tribunals to examine the case.Footnote ²⁸

It is significant that in most of the aforementioned decisions, the arbitral tribunals did de facto interpret CIL, as the VCLT was inapplicable. In the overwhelming majority of these cases, tribunals seemed to be unaware of this legal situation, as evidenced by the awards, which clearly indicate that they were applying the VCLT. This issue typically did not explicitly appear in the proceedings, as usually both the respondents and claimants seemed to also presume that the VCLT did apply after all.Footnote ²⁹ This was the case in disputes against Slovakia or the Czech Republic brought on the basis of 1990 BIT with the UKFootnote ³⁰ (A11Y, Anglia, Busta), the 1990 BIT with GermanyFootnote ³¹ (Binder, JSW Solar), or the 1992 BIT with the NetherlandsFootnote ³² (Eastern Sugar, Oostergetel, Achmea), as the Czech and Slovak Republics were only bound by the VCLT in 1993. The same issue related to the disputes against Poland (which acceded to the VCLT in 1990) on the basis of the 1988 BIT with AustriaFootnote ³³ (Strabag) and 1987 BIT with Belgium and LuxembourgFootnote ³⁴ (GPFFootnote ³⁵).

Conversely, the only cognizant approach in this respect in the cases involving intra-EU BITs was applied by the tribunal in EURAM on the basis of the 1990 Czechoslovakia-Austria BIT.Footnote ³⁶ Considering the argument of Slovakia that the VCLT was not applicable and that only the corresponding CIL, with respect to Articles 30 and 59 VCLT (but not Article 65), being applicable, the Tribunal referred to the exchange of diplomatic notes between the States concerned confirming the succession of the BIT in 1994. This assertion allowed it to recognise that the BIT was concluded in 1994.Footnote ³⁷ It is to be noted that the approach formulated by the tribunals on the same subject matter issue, where they decide incognizantly on the basis of CIL (such as Eastern Sugar, Oostergetel, Binder, or Achmea (I)), has been recognised and followed by arbitral tribunals directly adjudicating under the VCLT.Footnote ³⁸

When reaching the conclusion that the conflict of norms provisions do not apply as prerequisite of ‘the same subject matter’ is not fulfilled – arbitral tribunals most often did not offer any suggestions on how to solve the problem of a conflict of such norms. Initially, this approach was a consequence of not perceiving existence of such a conflict at all. Tribunals have also continued to maintain such a position after the judgment of the Court of Justice of the European Union (CJEU) in the Achmea case, despite the fact that the CJEU unequivocally confirmed the existence of such a conflict.Footnote ³⁹ Regardless, it is noticeable that since this judgment, arguments concerning the different material scope of the treaties has been increasingly and more clearly evoked by tribunals.Footnote ⁴⁰ Significantly, arbitral tribunals have pointed to the lack of ISDS in the EU Treaties as the main argument for the difference, while the CJEU explicitly stated that ISDS is contrary to the EU Treaties. A culmination of this legal reasoning was the 2019 ruling in Magyar Farming Company v Hungary in which the Tribunal, while noting that Article 30 VCLT was inapplicable due to the treaties’ differing subject matter,Footnote ⁴¹ determined:

In conclusion, the case law of arbitral tribunals makes it necessary to analyse Article 30 VCLT’s ‘same subject matter’ formulae in more detail. As it turns out, an element of this provision, overlooked even in the commentaries to the Convention,Footnote ⁴³ may be a key argument for rejecting application of this rule. This position, if comprehensively applied, could prevent any practical application of Article 30 VCLT and its corresponding CIL. Thus, from a systemic perspective, this line of interpretation should not be upheld,Footnote ⁴⁴ as it is difficult to assert that this potential interpretation of CIL presented by the arbitral tribunal would be followed by States or other international court and tribunals. The fact that approach of investment arbitration was clearly linked with the defense of its own jurisdiction by the tribunals has created circular or, perhaps, opportunistic lines of argumentation. This position, however, has been poorly embedded in general international law, which leads to a conclusion that its significance outside the framework of international investment law is limited. An alternative position would be recognising that this line of interpretation has led to significant gaps in conflict of rules under general international law.Footnote ⁴⁵

3 Subsequent Agreements in Relation to the Interpretation of Treaties

The second point under scrutiny concerns the subsequent agreements in relation to the interpretation of treaties regulated by Articles 31 (3)(a) VCLT. This issue was recently elaborated by the ILC in its 2018 Draft Conclusions on Subsequent Agreements and Subsequent Practice in Relation to the Interpretation of Treaties.Footnote ⁴⁶ Articles 31(3)(a) VCLT provide:

As has been stated by the ILC,Footnote ⁴⁸ and previously by international courts,Footnote ⁴⁹ tribunals,Footnote ⁵⁰ and investment tribunals,Footnote ⁵¹ the abovementioned rules also apply under customary law. Thus, these norms regarding interpretation could also be the object of interpretation as customary rules.Footnote ⁵² Such an approach has been applied by the WTO, where Article 31(3)(c) VCLT was considered to be custom in the Measures Affecting the Production and Sale of Clove Cigarettes case.Footnote ⁵³ Concerning the most-favoured nation (MFN) clause, the whole of Article 31 VLCT as a customary rule has also been scrutinised in investment arbitration jurisprudence.Footnote ⁵⁴

Already in 1966, the ILC had confirmed that the joint intention of parties, which underpins the conclusion of a treaty, has a particular authority when identifying the meaning of that treaty, even after its conclusion.Footnote ⁵⁵ This was in line with the ICJ’s jurisprudence.Footnote ⁵⁶ Regarding the legal effects of subsequent agreements, it is worth noting, first of all, Conclusion 7(1) adopted by the ILC, which states that

In the commentary to this provision, the Commission recognised, inter alia, the possibility for the parties to depart, by a subsequent agreement, from the ordinary meaning of the words of a treaty and to give them special meaning within the context of Article 31(4) VCLT. A departure from the ordinary meaning of words may be particularly justified if it is considered that the parties, in concluding the treaty, intentionally wished to give them an evolving meaning or content, by using general expressions, to take account of developments in international law.Footnote ⁵⁸

The ILC’s interpretation of Article 31(3)(a)-(b) VCLT can be compared to the position of arbitral tribunals on declarations or joint interpretations formulated by the parties to the treaty. One of the most famous examples of a subsequent agreement in international investment law is the interpretative note by the North American Free Trade Agreement (NAFTA) Commission,Footnote ⁵⁹ which narrowed the scope of Article 1105 NAFTA. In particular, the arbitral tribunal in Pope & Talbot v Canada stated that ‘were the Tribunal required to make a determination whether the Commission’s action is an interpretation or an amendment, it would choose the latter’.Footnote ⁶⁰ However, the tribunal found that this issue was not relevant as, regardless of the legal qualification of the note, its previous decision was consistent with it.Footnote ⁶¹ In the Methanex case, by contrast, the arbitral tribunal cited Oppenheim’s position according to which authentic interpretation of the parties to the treaty ‘overrides the ordinary principles of interpretation’.Footnote ⁶²

Besides the NAFTA Tribunal’s approach, which was based on an explicit clause in NAFTA, the issue of joint interpretation as a subsequent agreement has featured significantly in disputes concerning intra-EU BITs and intra-EU applications of the ECT.Footnote ⁶³ What is striking in the practice of tribunals in intra-EU disputes is that the readiness to accept any significance of such an interpretation is very limited, which differs from the position of the NAFTA tribunals. In the fundamental Eskosol decision, the Tribunal evaluated whether the Declaration of the Governments of the Member States on the Legal Consequences of the Judgment of the Court of Justice in Achmea and on Investment Protection in the European Union of 22 EU-Member States (EU Declaration)Footnote ⁶⁴ was a subsequent agreement under Article 31(3)(a) VCLT.Footnote ⁶⁵

The Tribunal criticised the lack of detailed justification in the EU Declaration, although it did not explain its source for such a requirement.Footnote ⁶⁶ It referred to the ILC’s works, although, surprisingly, not to those related to subsequent agreements, but instead its work related to reservations.Footnote ⁶⁷ Consequently, the Tribunal decided that the EU Declaration cannot be considered as a subsequent agreement as it does not refer to any particular provision of the ECT.Footnote ⁶⁸ Following the judgment of Singapore Court of Appeal in the Sanum case,Footnote ⁶⁹ the Eskosol Tribunal cited an excerpt from the 1966 Commentary of the Draft Articles on the Law of Treaties to assert that subsequent agreements are only intended to clarify the findings of the treaty negotiations.Footnote ⁷⁰ This element requires closer inspection as it has been repeated on several occasions by arbitral tribunals interpreting Article 31(3)(a) VCLT.Footnote ⁷¹

As a matter of fact, a broader citation of the ILC’s commentary in this respect does not in any way prejudge the accuracy of this position but rather (contrary to the conclusions drawn by the arbitral tribunals) emphasises in particular the significant nature of the subsequent agreements.Footnote ⁷² Furthermore, it is worth noting another extract from this decision: ‘VCLT Article 31(3)(a) is not, however, a trump card to allow States to offer new interpretations of old treaty language, simply to override unpopular treaty interpretations based on the plain meaning of the terms actually used’.Footnote ⁷³ This interpretation was subsequently cited by arbitral tribunals as rationale of their position in this respect, even when evaluating different legal situations.Footnote ⁷⁴ Thus, in the view of the Tribunal in Eskosol, a subsequent agreement ‘may “corroborate” or “support an interpretation that has already been determined by other methods,” such as “the objective elements listed in articles 31 and 32 of the Vienna Convention,” but it cannot override the application of those elements’.Footnote ⁷⁵ Finally, when the rights of individuals are impacted purportedly on the basis of general principles of law or CIL, the Eskosol decision supports the need for limitations upon the interpretative influence of subsequent agreements.Footnote ⁷⁶ This position had already been presented by the Enron tribunal; according to which, no new interpretation of an International Investment Agreement (IIA) can ‘affect rights acquired under the Treaty by investors or other beneficiaries’.Footnote ⁷⁷

Following the Eskosol decision, this restrained approach to subsequent agreements concerning the EU question (or so-called intra-EU objection) has been presented by other tribunals. According to the Addiko Bank Tribunal, EU member States do not have the right to interpret the TFEU, as this competence has been entrusted exclusively to the Court of Justice of the EU.Footnote ⁷⁸ The Strabag Tribunal stated that ‘[f]rom the text of Article 31(3) VCLT, it is evident that such “extrinsic” elements, while informative to the context of a treaty, cannot be used to rewrite the ordinary meaning of the text of the treaty under interpretation’.Footnote ⁷⁹

In Muszynianka, subsequent agreements relating to interpretation, although recognised as CIL,Footnote ⁸⁰ were considered as ‘merely one element’ that ‘[is] not an exclusive and dispositive method of treaty interpretation’.Footnote ⁸¹ Thus, the influence of subsequent agreements upon interpretation could be ruled out when, in the opinion of tribunal, it explicitly flows from the ordinary meaning.Footnote ⁸² The Muszynianka decision is extraordinary in this context as it totally ignored the bilateral declaration of the BIT’s State Parties, which the Tribunal justified by the fact the declaration was linked to the EU Declaration. Analysis of the latter was sufficient according to the tribunal.Footnote ⁸³

With regard to the relationship between the rights of individuals and interpretation on the basis of Articles 31 (3)(a) VCLT, the Tribunal in Green Power noted that the competence of States in this respect can induce inequality between parties of an arbitral proceeding, as one of them could change – with retroactive effect – the text of the ECT. Nevertheless, in this monumental decision, the Tribunal acknowledged that no such situation arises with respect to the EU Declaration in the context of the relationship between ECT and the EU law.Footnote ⁸⁴ Thus, what was crucial for the Green Power Tribunal was the interpretations made by the respondent and the investor’s home-State, and not that this interpretation was not confirmed by all the parties of both applicable multilateral treaties.Footnote ⁸⁵ Furthermore, the Tribunal, on one hand, seemed to support the position that subsequent agreements could not be applied retroactively if they related to the rights of individuals, but, on the other hand, stated that ‘Spain’s offer to arbitrate under the ECT is not applicable in intra-EU relations’ without indicating any concrete moment in time when this offer’s non-applicability took effect.Footnote ⁸⁶

As in the case of Article 30 VCLT – investment arbitration, in several cases, has interpreted Articles 31 (3)(a) VCLT and its customary counterpart. At least in one case (Strabag), only CIL was the subject of interpretation, as the VCLT was not applicable in the case. In this context it is to be noted that investment arbitration informs possible interpretations of Articles 31 (3)(a) in at least two areas – retroactive application and, connected with it, the impact on the rights of individuals.Footnote ⁸⁷ Thus, although ‘the parties to a treaty own the treaty and can interpret it’,Footnote ⁸⁸ their freedom in respect to international investment law is met with significant scrutiny and reticence from the arbitral tribunals. In fact: ‘[t]he only situations in which tribunals have been bound to follow the interpretive directions of the state parties are where the relevant treaty goes beyond the general VCLT rules and specifically provides for joint interpretations to be binding’.Footnote ⁸⁹

Such an approach tends to look at IIAs not only as a transaction between the parties,Footnote ⁹⁰ which have full discretion as to how to apply the mutually agreed upon provisions, but as an agreement which relates also to other beneficiaries. As Anthea Roberts puts it:

The problem with this line of reasoning is twofold. First, under international law, the State parties have the principal competence in the interpretation of treaties. This is not in any way changed by the fact that the interpretation can have a positive influence on their position in a dispute. It is still ‘their’ treaty.Footnote ⁹² Second, certainly, there is an accepted domestic practice of retroactive change in the interpretation of statutesFootnote ⁹³ or retroactive application of new precedents. The latter point has been evaluated by domestic constitutional and international courts and was, in principle, not found as violating the rule of law.Footnote ⁹⁴ Thus, domestic law does not have to justify its departure from the VCLT or customary law rules on interpretation. The powers of State parties to interpret ‘their’ treaties is well-represented in the China–Australia Free Trade Agreement (ChAFTA)Footnote ⁹⁵ which allows parties, in certain situations, to agree on whether the case pending before arbitral tribunal can be adjudicated due to the treaty constraints in this respect.Footnote ⁹⁶ As a result, there is an evident tension between the right of State parties regarding treaty interpretation under general international law and the emerging tendency for this competence to be limited with respect to rights of investors.

4 Conclusions

The two mentioned rules, on the conflict of norms and the influence of subsequent agreements regarding the interpretation of treaties, exemplify that investment arbitration can not only impact the interpretation of general international law concerning the responsibility of States, but they also have the potential to influence the law of treaties.

With respect to the first issue, it is to be noted that there exists a coherent line of tribunals decisions concerning relations between international investment agreements and EU treaties, which narrowly interpret the issue of ‘the same subject matter’ for possible conflicting treaty norms. Such an approach is particularly remarkable as it differs from the position taken by the ILC both in its work on the law of the treaties as well as in its study on the fragmentation of international law. Such a position differs also from the interpretations contained in two GATT/WTO decisions. Nevertheless, as this issue has not generally been the subject of scrutiny in international courts and tribunals in inter-State cases, investment awards cannot be disregarded as they represent the most extensive jurisprudence in this area. Still, from a systemic perspective, investment tribunals’ line of argumentation leads clearly to fragmentation as they approach investment treaties as almost self-contained regimes, which are resilient to a conflict of rules analysis unless the two applicable treaties are almost identical. It is remarkable that even a year after publication of the ILCs report on fragmentation, whose main message was ‘that the emergence of special treaty-regimes (which should not be called “self-contained”) has not seriously undermined legal security, predictability or the equality of legal subjects’,Footnote ⁹⁷ investment tribunals, at least with respect to the application of conflict of norms rules, have initiated a process leading towards the opposite direction. In this respect, in my view, investment awards which interpret not only rules of the VCLT, but also their corresponding CIL rules, seem to have pro futuro limited value and should rather be considered to have incidental significance for general international law.Footnote ⁹⁸ It seems that this approach taken by investment tribunals has been largely dictated by the need to defend their jurisdiction. What is striking is the number of cases in which tribunals were convinced that they could apply the VCLT when, in fact, only CIL was applicable in the respective case. This approach can also prove that at least with respect to rules with double force (treaty – customary) under international law, the method of interpretation can be similar to a large extent.

Regarding the VCLT and CIL rules on subsequent agreements several observations can be made. First, the ease with which arbitral tribunals reject interpretations formulated by States through subsequent agreements is striking. The analysis of these agreements is often rudimentary, absent, or the tribunal simply refers to other rulings – indicating that it considers these interpretations as their own – without sometimes paying attention to the difference in legal situations between the cases whose jurisprudence it cites. Second, tribunals have set out detailed requirements for the formulation of a declaration, without explaining the source for these requirements. Third, there is a considerable difference between tribunals referring to joint interpretations of State Parties, as envisaged by the treaty as binding (such as with NAFTA), and the tribunal assessing such interpretations through the lens of the VCLT or corresponding customary rules. With respect to the latter, tribunals tend to diminish the role of subsequent agreements by underlining the ‘taken into account’ formulae of Article 31(3) VCLT. Due to the laconic nature of the provisions of most of the investment agreementsFootnote ⁹⁹ (in particular so called ‘old-generation’ treaties),Footnote ¹⁰⁰ this approach enables investment tribunals, through the interpretation of the ‘ordinary meaning’ of terms and the ‘object and purpose’ of a treaty, to maintain its far-reaching interpretative power.

What follows from these observations is, to my mind, the necessity of bringing consistency between the case law of international investment arbitration tribunals and general international law, both as regards conflict of norms as well as of the role subsequent agreement and subsequent practices. Thus, there is a continuous need for more grounding of investment jurisprudence in the realm of general international law.Footnote ¹⁰¹

Still, investment arbitration can inform general international law in at least in two areas, which were not explicitly articulated by the ILC in its 2018 Conclusions, the retroactive application of interpretations and, connected with it, the impact on the rights of individuals. Certainly, in this respect, investment arbitration leans towards interpretations which restrain the rights of State parties as the ‘masters’ of treaties.Footnote ¹⁰² However, the line between rights of State parties and rights of the individual beneficiaries of the treaties (ie investors) has not yet crystallised. Thus, this trend coming from investment arbitration should be juxtaposed with the approach of other international tribunals and domestic constitutional courts’ jurisprudence.