This article widens the analysis of international organisations by including communist organisations, in particular the Warsaw Pact and the Council for Mutual Economic Assistance (COMECON). Drawing on archival research in Moscow, Bucharest, Berlin, Geneva and Rome, this article traces the origins, the evolution and the collective actorness of both organisations. Both COMECON and the Warsaw Pact went through a process of institutionalisation, reorganisation and multilateralisation and began to share many characteristics with their Western counterparts, such as the European Economic Community and NATO. Contrary to conventional wisdom these organisations thus developed into multilateral international organisations, which the other members could use to challenge Soviet unilateralism. Comparing COMECON and the Warsaw Pact with each other and with their Western counterparts, this article shows how these Eastern European international organisations contributed to shifting the balance of power within the Soviet Bloc by empowering their members as sovereign states and themselves as collective actors.