This paper argues that sociological analysis of law, and in particular the idea of ‘law in action’, can generate richer accounts of the role of law in EU integration than research which relies only on supranational and national legal systems’ own internal description of the legal field. A ‘law in action’ concept directs attention to intersections between a ‘legal’ and a ‘social’ sphere. While ‘law in action’ is often perceived in the literature as a normative order, this paper argues that variation in normative openness and closure is an important facet of the ‘law in action’. This argument is developed through an analysis of qualitative empirical data about the implementation of the EU Directive on Integrated Pollution Prevention and Control (IPPC 96/61/EC). The first part of the paper highlights limits of formal, instrumental and quasi-autonomous conceptions of law in EU integration. The second part develops the idea that ‘law in action’ is a conceptual device for analysing intersections between a ‘legal’ and a ‘social’ sphere. It also argues that political dynamics matter in such intersections. The third and final part of the paper draws on the data and suggests that norm closure is only sometimes achieved in determinations of what constitutes ‘the best available techniques’ (BAT). BAT is the key legal obligation which the IPPC Directive imposes on operators of mainly industrial installations.