This paper explores a neglected aspect of the wider debate about EU enlargement; namely bilateral disputes between a Member State and an applicant, where the former uses, or threatens to use, its membership to block membership to resolve a dispute. As we show through analysis of three cases - Italy and Slovenia, Slovenia and Croatia, and Greece and Macedonia - the EU’s transformative power does not always flow ‘outwards’ towards the state seeking membership. This raises interesting questions about enlargement as international bargaining between sovereign states filtered via a supranational entity formally responsible for the negotiations. Our cases suggest limits to the EU’s transformative power in the context of disputes that are linked to the meaning and significance of borders. When enlargement intersects with identity politics, the result can be potentially destabilizing in ways that can lead to a decline in the EU’s legitimacy. It is not surprising that the Commission prefers disputes to be resolved bilaterally or via a third-party.