This article examines the role of Europarties in the European Union (EU) institutional and constitutional or treaty reform, in decisions and negotiations leading to the adoption of treaties in the 1980s and 1990s. The existing literature on such reform in the EU largely overlooks the role of Europarties in the making of new treaties. Research on EU treaty reform usually operates within a state-centric ontology and framework for analysis. Challenging previous analyses and moving beyond state-centrism and intergovernmentalism, strictly inter-state bargaining, this article offers a complementary transnationalist account of what is happening in the drama of grand bargains or history-making treaty negotiations in the EU. There is a transnational dimension to such treaty reform; there is Europarty mobilization and influence. In conclusion, Europarties matter when they are in numerical ascendance, relatively cohesive and able to mobilize their networks of political parties and leaders.