Heterogeneity with respect to underlying country characteristics can have diverse effects on the formation of international environmental agreements. Given that such agreements can be seen as coalitions forming to provide an international public good, relevant dimensions of heterogeneity are, for instance, countries’ endowments, provision costs and the valuation of the respective public good. Heterogeneity with respect to any single one of these dimensions weakly decreases the size of a stable agreement. However, taking into account a second dimension of heterogeneity can again lead to an increase in the size of a stable agreement. Furthermore, heterogeneity might influence the political process through giving different political weight to countries. Such heterogeneity over political power can impact the stable size of a coalition in a positive or negative way, depending on the characteristics of the countries having the larger weight.