Most of works dealing with the design of international commitments focus on the rationality of actors which allows them, among several options, to choose the one that responds optimally to their interests. Although these works represent a significant contribution to the analysis of states’ behavior within international institutions, they neglect the role of intersubjective structures in the formation of states’ interests. Curiously, there are few theoretical works that emphasize this epistemological limit of rationalist design of international commitments. Moreover, few case studies focus on this. The central purpose of this study is to show that while the design of international commitments is a rational choice on the part of states, this choice is primarily conditioned by states’ identities. The case study is Chile’s decision to join Mercosur as an associate member.