This article investigates the efficacy of international environmental agreements (IEAs). It reviews approaches from the political sciences and from economics to measure the efficacy of historical IEAs. It is argued that the current literature is deficient in many respects but that by appropriate extensions an "ideal" method of measurement could be constructed, conceptually. However, due to prohibitive information costs this ideal method is not employed in this article. Instead, we use a simple "theoretical pre-check": We design a basic model which we believe captures the main forces at work in international pollution control. With respect to all possible assumptions underlying this model we choose those which are the most favourable ones regarding the stability and efficacy of an IEA. For these ideal assumptions we derive a necessary condition for the effectiveness of an IEA. Subsequently, we check a large set of existing IEAs to find that no IEA satisfies this condition. This result leads us to discuss the question why IEAs are signed at all if they do not depart from the status sine pacta. It is public choice theory which helps to solve this puzzle.