Starting from principal-agent theory, this article analyses the conditions under which an EU negotiator enjoys a particular degree of negotiation autonomy vis-à-vis the member states he represents during multilateral environmental negotiations. A Qualitative Comparative Analysis of eight EU decision-making processes with regard to international negotiations leading to a multilateral environmental agreement indicates that the compellingness of the international negotiations explains the occurrence of negotiation autonomy. However, the international compellingness does not provide explanatory power to understand the particular degree of negotiation autonomy. To understand when an EU negotiator enjoys a high degree of negotiation autonomy, variables such as preference distances, information asymmetries and institutional density need to be taken into account.