Our paper deals with the transformative impact of enlargement on Regional Organizations (ROs) in a comparative perspective. Against the backdrop of the ‘deepening versus widening’ debate we argue that accession demands are likely to trigger processes of norm and identity contestation inside ROs. More specifically, in order to react to an accession demand a RO needs to assess the compatibility between itself and the candidate state. This opens windows of opportunity for norm entrepreneurs to contest the RO’s existing norm apparatus and to bring democracy or human rights issues on the agenda. We test our theory by conducting comparative case studies of enlargement processes in ASEAN, the EU, and Mercosur. Our within- and across-cases analyses underline that there is evidence for the mechanisms postulated by our theory. In addition, they specify under which internal and external conditions such processes can contribute to norm change inside of ROs.
