Cooperation in international organizations (IOs) is sustained by the socialization of state agents and their internalization of the organizations’ norms and identity. This article builds on a structured comparison of the scope conditions for socialization among permanent representatives in two organs of the European Union and the North Atlantic Treaty Organization—the Political and Security Com-mittee (PSC) and the North Atlantic Council (NAC). In this study, we present some unexpected findings: First, the NAC is experiencing greater internalization (stronger socialization) than the PSC, normally held as a critical case of international sociali-zation. Second, unambiguous norms favour socialization to a larger degree than ambiguous norms, refuting a widely held assumption about the pro-internalization effect of diffuse norms. Given that member states seem to grant their representa-tives larger “room for manoeuvre” when the norms of the IO have material stakes, the socialization effect of an IO’s norms is dependent on the perceived utility of the organization’s mission.