The international system is increasingly characterized by the presence of partially overlapping institutions that regulate and monitor states’ performance in a given area, known as regime complexes. Whereas scholars have identified a set of possible consequences that regime complexity may have on both the institutions and the actors participating in these institutions, we still lack a comprehensive analytical model to assess the extent to which these consequences actually take place. This article devises a model to assess the extent to which institutions within regime complexes strengthen or undermine each other. It applies the model to the United Nations’ Universal Periodic Review and to the process of examination of state reports by the United Nations treaty bodies. Data were collected by means of 22 semi-structured interviews conducted with actors directly involved in the mechanisms as governmental officials, United Nations secretariat staff, members of the treaty bodies, and civil society activists. Findings reveal that overlapping activities by different treaty bodies appear to be much more problematic than those occurring between the UPR and the treaty bodies.