This article analyzes the effectiveness of transnational multi-stakeholder partnerships for sustainable development — also known as “Type II outcomes” of the 2002 World Summit on Sustainable Development — in the sustainable energy sector. We combine quantitative and qualitative research. Quantitatively, we use a database of 340 partnerships, including 46 partnerships that focus on energy. Our qualitative analysis includes case studies of ﬁve partnerships that appear as the most effective and ﬁve that are operational but only with modest degrees of effectiveness. We study two competing hypotheses. The ﬁrst, rooted in institutionalism, assumes that variation in effectiveness is related to organizational structures and procedures. The competing hypothesis emphasizes the power of actors and expects partnerships that involve key business actors and powerful Northern states to perform better. We conclude that the level of institutionalization is most important in explaining effectiveness, while powerful partners and the type of internal organization may further enhance effectiveness.