Does the European Union (EU) need to be a cohesive actor internally in order to be effective in global trade governance? And is internal cohesiveness a necessary and sufficient condition for the Union’s external effectiveness? Even though these questions are central to explaining the EU’s effectiveness, only few studies have dealt with it. In this contribution, I argue that internal cohesiveness is not a sufficient condition for EU external effectiveness. The bargaining configuration, determined by bargaining power symmetry or asymmetry, equally conditions effectiveness. This argument is illustrated by two case studies with varying bargaining power. The findings demonstrate, first, that high internal cohesiveness and bargaining power asymmetry can lead to high effectiveness, as in the EU–Mexico negotiations. Second, they show that, under a relationship of bargaining power symmetry, high cohesiveness is not a sufficient condition for external effectiveness to occur, as is evident in the Doha round.