-1- The need for appropriate short run design of mechanism in order to achieve long run effectiveness in international environmental problems. Urs Steiner Brandt. Keywords: Design of international environmental agreements, endogenous decision process, short run mechanisms, characteristics of international environmental problems. Abstract: A general model is presented that enables us to point to which mechanism has the highest likelihood of being accepted by the participating countries in the initial stages of the negotiation phase in international environmental problems, and hence to determine the optimal path to reach an truly effective agreement in as short a time span as possible. The point of departure is that in the international society no supranational agency exists with sufficient power to implement a given order as long as it is not based on voluntarily agreements. For international environmental problems this has as a consequence that choice of regime will be made by the same agents (= countries) that afterwards act in this regime. The model seeks to in cooperate this by use of the fact that each international environmental problem has a specific set of characteristics, and only a full description of this set enables us to determines which mechanism should be applied. This result hinges on the fact that no mechanism that is political feasible is superior for all combinations of characteristics because each mechanism has trade-offs especially between fairness and cost efficiency and between these two and non-manipulability. Acknowledgement: Earlier versions of this paper were presented at the “environmental lunch seminar” at Tilburg university, at the “research seminar” and the “seminar for sustainable economy”, both at the department of economics, the Business School in Aarhus. Helpful comments from the participants are gratefully appreciated. Address: Urs Steiner Brandt, Department of Economics, Aarhus School of Business, Fuglesangs Allé 20 DK-8210 Århus V, Denmark. e-mail: URS@HDC.HHA.DK.