A trinity composed of legally binding regulations, an independent financial mechanism, and a compliance mechanism characterizes the institutional design of the Minamata Convention on Mercury. Meanwhile, few existing environmental treaties feature an independent financial mechanism as well as a compliance mechanism. Why did the Minamata Convention acquire two mechanisms? There are two rival hypotheses on uncertainty about institutional consequences and international agreements. The rational design school posits that countries can predict institutional consequences by acquiring all pieces of relevant information (Koremenos et al. 2001) and views the trinity as a rational design to enhance developing countries' regulatory capabilities under strict compliance. In contrast, the institutional diffusion school assumes that countries have limited information-processing abilities and use cognitive heuristics in designing institutions and argues that countries designed the trinity by learning from existing cases (Ovodenko and Keohane 2012). In this paper, I compare the negotiations process of the Minamata Convention with that of the Stockholm Convention on Persistent Organic Pollutants (POPs). To test the hypotheses, I examine how countries resolved informational uncertainty in both negotiations by utilizing negotiations records and personal interviews with key officials as data. The analytical results support the institutional diffusion hypothesis by indicating that the trinity within the Minamata Convention is a product of countries' heuristic and incremental learning from existing treaties. for their invaluable comments. I wish to thank my interviewees for their insights about the negotiations on the Minamata Convention and the organization of the UNEP. Compliance with ethical standards: The author declares no conflicts of interest.