The member states of the European Union (EU) have transferred their sovereignty over trade policymaking to the supranational level. When entering into trade negotiations with third countries, they must first reach a common bargaining position among themselves and later defend that position with a “single voice” at the international table. How do the institutional rules, through which the fifteen different voices are aggregated into a single one, affect international outcomes? Differentiating between a “conservative” and a “reformist” negotiating context, I argue that voting rules and negotiating competence in the EU determine both the probability that the negotiating parties conclude an international agreement and the substantive outcome of the negotiations. The recent EU–U.S. trade negotiations on agriculture, public procurement, and open skies are all evidence that, for a given distribution of preferences, internal EU institutional mechanisms affect the outcomes of international trade agreements.