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This paper analyzes the shaping of international environmental agreements based on two case studies: the negotiations surrounding the reduction of air pollution and the preservation of the ozone layer. It takes in account the positions of countries, how influence is exerted, and the roles played by science and the institutions that led the negotiations. Three main findings arise from this analysis. First, the definition of an agreement can result from commitment by one country or a small coalition of countries. Second, in a context of uncertainty, one country (or a small coalition of countries) can be brought to adopt a unilateral policy so as to place pressure on its partners to do likewise. Depending on the countries’ interests, science may be used as an argument to justify participation or withdrawal. Third, the executive director of the organization leading the negotiations can play a decisive role in reaching consensus.