What drives processes of institution building within regional international organizations? We challenge those established theories of regionalism, and of institutionalized cooperation more broadly, that treat different organizations as independent phenomena whose evolution is conditioned primarily by internal causal factors. Developing the basic premise of ‘diffusion theory’ — meaning that decision-making is interdependent across organizations — we argue that institutional pioneers, and specifically the European Union, shape regional institution-building processes in a number of discernible ways. We then hypothesize two pathways — active and passive — of European Union influence, and stipulate an endogenous capacity for institutional change as a key scope condition for their operation. Drawing on a new and original data set on the institutional design of 34 regional international organizations in the period from 1950 to 2010, the article finds that: (1) both the intensity of a regional international organization’s structured interaction with the European Union (active influence) and the European Union’s own level of delegation (passive influence) are associated with higher levels of delegation within other regional international organizations; (2) passive European Union influence exerts a larger overall substantive effect than active European Union influence does; and (3) these effects are strongest among those regional international organizations that are based on founding contracts containing open-ended commitments. These findings indicate that the creation and subsequent institutional evolution of the European Union has made a difference to the evolution of institutions in regional international organizations elsewhere, thereby suggesting that existing theories of regionalism are insufficiently able to account for processes of institution building in such contexts.