Analysts of the European Union (EU) and international bargaining have generally failed to appreciate how the shift within the EU from unanimity to qualified majority voting has affected European bargaining positions and international outcomes. I analyze the international effects of changes in EU decision-making rules with a simple spatial model and assess the utility of the model in two cases of environmental bargaining that span the entry into force of the Maastricht Treaty. The EU can decisively shape international outcomes by concentrating the weight of its fifteen member states on a single substantive position and rendering that position critical to any internationally negotiated agreement. The findings generalize to numerous areas of EU external relations and suggest that analysts should attend specifically to the EU and more generally to domestic and regional institutional factors in explaining international bargaining outcomes.