What explains the variation in institutional adaptation of national parliaments to European integration? Whereas the existing literature has mainly focused on domestic conditions, this article explains institutional adaptation to integration by focusing on inter‐parliamentary diffusion. The argument draws on ‘learning’ mechanisms of diffusion on the demand side and on ‘emulation’ mechanisms on the supply side. Parliamentary demand for external inspiration is related to uncertainty about functional oversight institutions, and the selection of sources to perceptions of similarity and success. Demand arises in new European Union member parliaments and young democracies that then turn towards culturally alike countries and old democracies. Using spatial econometrics, support is demonstrated for the argument in the article while ruling out alternative diffusion mechanisms such as spatial proximity and learning from Scandinavian frontrunners once links along cultural similarity and democratic experience are controlled for. The results underline the limits of the ‘isolated polity’ approach in the comparative study of institutions in Europe's closely integrated political system, while also showing that, even in this favourable environment, diffusion pathways are contingent on the mechanisms generating demand among policy makers and shaping their selection of sources for external information.