This article considers the entanglement of theories of international politics and international economics on the one hand and an emergent supranational policy actor on the other. The formation of the European Communities in the late 1950s and early 1960s is well known as a moment of radical supranational institutional creation. This is a period in which both economists and political scientists sought to theorize the processes of regional integration as it was happening. The article shows that the newly-formed European Commission, in both its communicative and coordinative discourse, drew selectively on the ‘live’ knowledge archive of both international economics and international relations to generate a strategic narrative about what European economic integration entailed, how it would be accomplished and (crucially) what kind of actor it was. At the same time, the semi-inductive quality of academic knowledge production meant that the Commission’s activities were simultaneously being theorized into the live archive from which it was drawing. The paper examines the influence of two bodies of theory: one from economics (Balassa’s theory of international economic integration), and the other from political science (neofunctionalism). The former is a striking influence on early discussions, initiated by the Commission, on monetary union, but its finger prints are rather more enduring — being visible in Commission communicative discourse well into the 1970s.