Interregional co-operation, defined as a bilateral dialogue between regional organisations, has become a familiar phenomenon of international politics. Although the forms and functions of this type of co-operation have been discussed in depth, little is yet known about the causes of divergent degrees of effectiveness as measured according to the objectives set by the relevant actors themselves. This contribution presents an interpretative and comparative study of three cases of interregionalism: ASEAN-EU, EU-MERCOSUR and MERCOSUR-ASEAN. Regional organisations' capacity to act - as defined by their organisational structures, decision-making structures, and resource structures - can be shown to correlate with the effectiveness of their interregional dialogues. Furthermore, a minimum level of capacity is required on both sides in order to achieve any joint objectives at all.