This article lays out a framework of authority for the analysis of non-binding instruments of international organisations as exercises of international public authority. This framework considers influence on freedom of addressees and lawappliers, rather than formal bindingness, to be the decisive criteria of authority. I argue that this approach can better explain the legal relevance and impact of these instruments on the conduct of different actors, particularly in the field of international environmental law. The article further proposes that the decisionmaking processes taking place inside international organisations, which are a distinctive feature of this form of governance, influence the authority of the instruments adopted. These processes have, at least, three characteristics that are particularly relevant in relation to environmental regulation: (i) the scientific and technical expertise of the organisation and other participants, (ii) the range of participants and the achievement of broad consensus within relevant communities of practice, and (iii) the development of procedural norms necessary to implement and comply with existing obligations. Finally, this framework of authority is applied to analyse the authority of the instruments adopted by the Food and Agriculture Organisation on the regulation of sustainable fisheries. This case study illustrates the explanatory value of the framework and draws conclusions on its implications for the law of international organisations.