What are the calculations of states in the international system when it comes to cooperation in the area of prohibition? By shifting the focus from normative policy outcomes to the importance of relationship-specific negotiation incentives, this work helps to explain the emergence and stability of international regimes in terms of the regulation of drugs and small arms/light weapons (SALW). It develops a model which focuses on the interplay between transaction and governance costs in order to explain the varying extent of cooperation we observe in international prohibition.