Cooperation in international organizations is to a large degree driven and sustained by socialization – the process of inducting actors into the norms and rules of a given community. In the context of international organizations, the most influential state agents are the permanent representatives, the member states’ ambassadors to an international organization. However, systematic studies on the conditions for the socialization of permanent representatives are conspicuously absent in the literature. Instead, most previous research has focused on the conditions for socialization of “high-level officials” in the European Commission. This quite narrow perspective has led to a “N=1” problem and the generalizability of these studies have suffered as a result. In this thesis, I have aimed to broaden this perspective by testing six hypotheses on the conditions for socialization within two committees of permanent representatives in two different international organizations, the PSC (EU) and the NAC (NATO). This has been done by conducting elite interviews with 21 permanent representatives and deputies in Brussels. In contrast to previous research, the results of this thesis suggest that four of the six tested hypotheses should be disregarded, while two hypotheses – the representative’s relation to its MFA and the ambiguity of the international organization’s norms– should be given more theoretical consideration in future research. In addition, the interviews have shown that the degree of “internalization”, which can be understood as the goal and outcome of socialization, is stronger in the NAC than in the PSC. This outcome also contests the conventional wisdom that the EU is a sui generis case of socialization. On the basis of these results, I argue that the socialization of permanent representatives in international organizations is likely to occur if the organization’s norms have unambiguous, existential and materialistic consequences.