International environmental negotiations often involve conflicts between developed and developing countries. However, considering environmental cooperation in a North-South dichotomy obscures important variation within the Global South, particularly as emerging economies become more important politically, economically, and environmentally. This article examines change in the Southern coalition in environmental negotiations, using the recently concluded Minamata Convention on Mercury as its primary case. Focusing on India and China, we argue that three key factors explain divergence in their positions as the negotiations progressed: domestic resources and regulatory politics, development constraints, and domestic scientific and technological capacity. We conclude that the intersection between scientific and technological development and domestic policy is of increasing importance in shaping emerging economies’ engagement in international environmental negotiations. We also discuss how this divergence is affecting international environmental cooperation on other issues, including the ozone and climate negotiations.