A growing literature on diffusion shows that ideas, policies and institutions originating in the European Union shape decision-making in other regional organisations (ROs). However, a research programme which claims to examine processes of interdependent decision-making between ROs in general cannot treat a single RO as the ‘default’ source of this diffusion. This paper thus (1) discusses what we term the ‘multiple sources’ problem in diffusion research and (2) presents a model of institutional diffusion, which highlights the important role of local actors in the multiple sources scenario. Based on original data collected via extensive fieldwork in East Africa, we present a case study on the interdependent sources behind the East African Community (EAC)’s establishment in drawing on this model. We find that EAC policymakers selected between, combined and modified several external institutions. These findings highlight loc