European Union (EU) enlargement has been somewhat neglected theoretically in studies of European integration despite its importance both for member states, applicant states and the EU itself. The main purpose of this dissertation is to make a theoretical contribution to the literature through applying an institutionalist analysis to the attitudes of European Commissioners and member states as expressed through their speeches. The major research question of this dissertation is: „Which theory of institutionalism, rationalist or constructivist/sociological, better explains the attitudes of European Commissioners and member states towards deepening relations with Poland and Turkey?‟ In order to answer this question, the different stages of the EU‟s Eastern enlargement are analyzed through the case studies of Turkey and Poland. The results of this analysis demonstrate that the logic of consequentialism rather than logic of appropriateness has prevailed in the formation of the attitudes of both European Commissioners and member states towards Poland and Turkey.