The Eastern enlargement of the European Union was accompanied by the Eastern enlargement of the European party federations ("Europarties"). On the basis of a comprehensive theoretical model the study investigates the influence of the biggest Europarties EPP and PES on their Hungarian, Romanian and Slovak partner parties from 1989 to 2012. The methods and data used are content analyses of manifestos and party statutes, interviews and archival documents. The main finding is that Europarties play a crucial role in enlargement processes of the EU on the level of political parties. Both Europarties exerted strong influence on their CEE partner parties. As a result, the CEE parties became more similar to the established West European member parties of the Europarties regarding their strategic behaviour, programme, and intra-party democracy.