This article focuses on the way the European Union acted as a negotiating party during the international negotiations leading to the Stockholm Convention on Persistent Organic Pollutants (1998)(1999)(2000). Starting from a principal-agent model, the article discusses how the EU participated in these negotiations and how the internal decision-making process developed. It argues that the EU was able to negotiate in a unifi ed and infl uential way by defending a common position, which was expressed by a fl exible negotiation arrangement, at the international level. Three features of the EU decision-making process engendered such a strong EU negotiation arrangement: homogeneous preferences among the actors in the EU, symmetrically distributed information among them and a cooperative and institutionally dense decision-making context.