International law increasingly obliges states to negotiate in good faith environmental disputes that arise in connection with the use and protection of shared or common property natural resources — that is, watercourses, fisheries, and migratory species. Articulation of this duty to negotiate in good faith has been vague, and, perhaps as a consequence, disputes have been protracted or have gone unresolved. Part of the problem may be that states do not know how to interpret their competing rights in the resource. This article explores the facilitative potential of international authoritative soft law to good faith negotiation where rights and obligations of resource use and protection are broadly stated and their relationship to one another is unclear. In this context, our understanding of the relevance, sources, and use of law in the negotiation process contributes to whether law functions to facilitate or frustrate dispute resolution. Through discursive interaction undertaken in good faith, states should look to international authoritative soft law to explicate, integrate, and reconcile their legitimate interests within their competing rights and obligations, according to prescribed legitimacy criteria.