Abstract This article argues that European integration has triggered a dual constitutionalisation process in Europe. One is the revision of national constitutions to accommodate the integration project at the national level. The other is the construction of transnational rules to regulate novel inter-state relationships at the European level. EU referendums are contextualised in such a duel constitutionalisation process. At the domestic level, EU referendums handle the debates on national constitutional revision. At the transnational level, these popular votes ratify supranational constitutional documents. The article comparatively analyses three types of EU referendums-membership, policy and treaty referendums-according to this analytical framework, exploring the campaign mobilisation of voters, national governments, and transnational institutions, and examining the legal and political interaction between referendums and European integration. A key finding is that, as the dual constitutionalisation process deepens and widens, entrenched domestic players and restrained transnational actors are under increasing pressure to voice themselves in EU referendums.