The recognized benefits to a hegemon of working through an intergovernmental organization (IGO) include legitimating its policy preferences, disseminating them with efficiency, and promoting stability. While most would agree that international law is important in this process, it is less easy to map exactly how international law fulfils this role. Using the cognitive structures of co-operation (CSC) approach to the political interpretation of multilateral treaties, this article demonstrates at a relatively low level of abstraction the way in which a constitutive treaty embeds an ideational structure integral to the political relationships within the IGO. This can serve the interests of the hegemon but may also make it difficult for the hegemon to disseminate a fundamentally changed policy should its preferences alter. This paper uses the CSC theory of treaty interpretation to trace the under-recognized role of the United States in bringing about the 1982 adoption of a moratorium on commercial whaling by the International Whaling Commission.