The European Union (EU) has made effective corruption control a condition for membership, but it cannot sanction non-compliance once a country has joined. The Cooperation and Verification Mechanism (CVM) was an institutional experiment to compensate the loss of sanctioning power after accession with continued monitoring. Most commentators dismiss the potential of such monitoring without enforcement. This article’s original coding of the CVM reports with regard to corruption control in Romania and Bulgaria provides an empirical basis to assess the CVM’s ability to foster compliance. It suggests that monitoring can have a positive impact on state compliance even without material sanctions: despite the low expectations in the literature, compliance in Romania was significantly better than in Bulgaria. We explain Romania’s better compliance record with successful domestic institution-building. In contrast to Bulgaria, Romania created strong anti-corruption institutions that served as a powerful institutional base for the fight against corruption. The CVM has not only had a direct effect on institution-building, but also an important indirect effect. As the anti-corruption institutions remain vulnerable to governmental interference, the CVM played a key role as a social constraint on attempts by the government to curb their power and as a focal point for societal mobilisation.