As community-based organisations, regional international organisations (RIOs) entail clusters of community norms that define appropriate behaviour within the region and constitute regional identities by setting standards of legitimate membership in the RIO. Existing research has shown that such community norm clusters are constantly contested. We argue that states’ threats to withdraw from RIOs provide valuable focal points for analysing the occurrence and effects of severe instances of contestations of RIOs’ community norms. Based on a series of qualitative case studies, this chapter makes three contributions. First, we analytically distinguish between the contestation of norms defining the rights or duties of membership in community organisations and between the contestation of norms’ substantive content or applicatory scope. Second, we develop expectations on the frequency of occurrence of different types of norm contestation and their effects. We argue that the applicatory scope of community norms is more frequently contested than their substantive content and that this occurs more often concerning membership duties than membership rights. Third, the case studies indicate that different forms of contestations have different effects on norms’ stabilisation, modification, or replacement. We conclude by outlining pathways for further research at the intersection of norm contestation and comparative regionalism.