Negotiations in the Council of the European Union take place in an environment that is relatively unstructured by formal rules and procedures. Indeed, most of the negotiations take place in working parties and committees composed of national officials and diplomats that ‘prepare’ the Council’s work. In other words, decisions are being made in fora that are not recognized by the European Treaties, by government representatives that possess no formal powers to do so, and according to unwritten rules and procedures that have largely developed through practice over time rather than intentional design. Even though formal provisions, such as the voting rule, may have indirect effects on negotiators’ behaviour, they often do not reflect decision-making in practice. To shed more light on the negotiation process in the Council, this paper presents a quantitative case study of negotiations on the Air Quality Directive. To capture the complexity of multi-issue, multi-actor negotiations over time, the study describes the negotiation process as a series of two-mode networks consisting of member states and their negotiation positions. The statistical analysis focuses on two aspects of this process, the number of negotiation positions stated by member states and the propensity of those positions being incorporated into the legal text. The results suggest that larger countries state more negotiation positions than smaller ones, and that the size of the group supporting a negotiation position is an important determinant in explaining its incorporation into the final agreement. Furthermore, these relationships are stable across stages of the negotiation process and different decision-making fora of the Council, suggesting that negotiation behaviour in informal settings is not qualitatively distinct from negotiation behaviour in more formal arenas.