This article analyses the EU's influence on regional institutional change in Mercosur and the Southern African Development Community from a diffusion perspective. Focusing on market-building objectives and dispute settlement mechanisms, it addresses the puzzle that policy-makers in both regions have, over time, increasingly adopted EU-style institutional arrangements even though alternative institutional models more suitable to their preferences for ‘pragmatic’, sovereignty-preserving cooperation have been available at various critical junctures of institutional evolution. The article makes two main arguments. First, it suggests that EU influence has affected outcomes in several specific ways that are irreducible to, and quite different from, mainstream functional accounts of economic regionalism. Second, it contends that the diffusion of EU institutional templates can be understood as a process of spurred emulation, when regional policy-makers emulate EU institutional models under conditions of uncertainty and promoted by EU-oriented domestic actors as well as the EU's direct involvement in the process.