In global environmental governance, numerous new international organizations have emerged from dozens of multilateral treaties signed over the last 30 years. This paper focuses on intergovernmental organizations in an organizational theory perspective with a particular focus on organizational learning processes. It explores where and when international organizations exhibit organizational learning with significant effects on the organizations' internal structure and behavior. Key hypotheses from principal-agent theory and organizational learning theory are tested in eight case studies of international organizations involved in global environmental governance. The analysis shows that organizations engage in three forms of learning: reflexive learning, adaptive learning, and no learning. Explanations of the observed variation depend on specific learning mechanisms, change agents in leadership functions and external triggers such as pressures from governments or nongovernmental actors.