This article explores the conditions for Europarty influence. Europarties can be expected to matter when they are in numerical ascendance, relatively cohesive and able to mobilize their networks of political parties and leaders. The explanatory power of this argument is tested empirically through an in-depth case study: the role of the centre-right European People’s Party (EPP) in the negotiations leading to the adoption of the Amsterdam Treaty in 1997. The article submits and documents that there is a transnational dimension to such treaty reform, that there is Europarty influence but clear limits to such influence. The case demonstrates how factors pertaining to domestic politics, especially in Germany where chancellor Kohl was severely constrained in the search for a compromise over the new treaty, limited the scope for the EPP to more significantly shape the outcome of the treaty negotiations. Accordingly, domestic political context condition Europarty influence.