International organizations (IOs) and social movements interact in various ways. In the last years, IOs have created numerous opportunities for civil society to issue their criticism. While such strategies of openness may lead to the increased inclusion of movements’ concerns and legitimate IOs, they do not always have the intended effect to deflect and deradicalize criticism. From existing studies, we know that attempts to impede radicalization do not always work: IO’s inclusion and cooptation strategies often have a dividing effect on social movements that stimulates parts of the movements to radicalize. Until now, there are only few systematic assessments on this relationship, though. How do social movements react to IOs’ diverse cooptation strategies? Drawing on extensive comparative research on different activist networks involved in the Global Justice Movement in Europe and Southeast Asia, this paper analyzes pathways of radicalization in light of ‘cooperative’ IO behavior. Based on a relational approach we argue that the mode of interaction between IOs and social movements is important in shaping movement strategies and action. We identify the conditions under which IOs’ cooptation strategies can spark off radicalization processes in social movements taking into consideration their interaction with other factors such as movements’ (perceived) alternative opportunities at national and local contexts as well as processes of internal deliberation.