This article explores a neglected aspect of the wider debate about EU enlargement: bilateral disputes between a member state and an applicant, where the former uses, or threatens to use its membership status to block the applicant's progress in order to resolve a bilateral dispute. Through analysis of three cases -Italy and Slovenia, Slovenia and Croatia, and Greece and Macedonia -we show that the EU's transformative power does not always flow 'outwards' towards the state seeking membership. This raises interesting questions about enlargement as a process of international bargaining between sovereign states filtered via a supranational entity formally responsible for negotiations. The cases suggest limits to the EU's transformative power in the context of disputes that are linked to the meaning and significance of borders. It is not surprising that the European Commission prefers disputes to be resolved bilaterally or via a third party.Keywords: borders; European Union; enlargement; South East Europe 'Those who knock on Europe's door such as Ljubljana must make a gesture of repentance.' (Gianfranco Fini, quoted in Corriere della Sera, 1994, p. 5) Studies of EU enlargement typically focus on the relationship between the European Commission as an agent of the member states and the applicant country. Central to EU enlargement is that member states collectively consider a country's application and that progress (or not) is determined by satisfying the EU's criteria as a collectivity -a process concerned with satisfying a supranational interest, not national criteria. It is then assumed that the EU's transformative power flows outwards from the member states (the 'insiders') to the applicant (the 'outsiders'). We identify a gap in the accounts offered by these existing studies, which is the impact of domestic mobilisations about the status and meaning of borders on EU enlargement.The role and impact of bilateral disputes is a neglected aspect of analyses of the relationship between a member state and an applicant, where the former uses, or threatens to use, its membership to block membership in order to resolve a dispute. Our analysis takes three cases from South East Europe (SEE) -a region that is central both to the EU's recent history and future development. We show how the 'unfreezing' of border conflicts in four countries (Croatia, Italy, Macedonia and Slovenia) linked to the break-up of the former Yugoslavia and in the context of European integration produces outcomes that confound the usual expectations about the direction of transformative power -namely outsider adjustment. We demonstrate three other potential outcomes: insider adjustment, mutual adjustment and stalemate. Each of our three case studies flows from the complexities of Balkan politics but are now framed by European integration. All were triggered by Yugoslavia's break-up and demonstrate unexpected outcomes if the assumption were that bs_bs_banner