This article addresses the role of party elites in the setting of European multilevel governance and transnationalization. Emphasis is placed on informal networks, elite socialization and policy coordination. The analysis focuses on the 1985 Intergovernmental Conference, which led to the Single European Act (SEA), and the political family of Christian Democrats, most notably party leaders' meetings. The research material consists of interviews and archival sources. It can be shown that the SEA was to a large extent shaped through transnational party elite socialization. Challenging previous analyses, there is sufficient documentary evidence to claim that political parties, or rather party elites, were centrally involved in the making of this historic treaty. Such involvement of elected political representatives could further the democratic legitimacy of the European Union, but reinforces problems of intra-party democracy