A comparison of the results of the six most recent Intergovernmental Conferences (IGCs) in the European Union (EU) indicates that member governments' success in achieving substantial compromises based on issue linkages differs across cases. An examination of supranational and intergovernmental bargaining theory shows that both fail to provide a satisfactory explanation for this variation. Instead, we argue that the problem that all participants to a negotiation have an incentive to maximise individual rather than overall gains often leads to efficiency losses. In IGCs, EU member states established two procedures to surmount this problem: a preparatory phase precedes the actual negotiations, and the Council presidency acts as a mediator in the negotiations. Lack of time for preparation, a biased presidency, or external shocks that reduce the capacity of the presidency to guide the negotiations, however, can cause efficiency losses in IGCs since under these conditions the two procedures fall short of facilitating bargaining efficiency. In an empirical analysis of all IGCs from the Single European Act to the Constitutional Treaty, we find support for our argument.