This paper extends the standard model of self-enforcing dynamc international environmental agreements by allowing the length of the period of commitment of such agreements to vary as a parameter. It analyzes the pattern of behavior of the size of stable coalitions, the stock of pollution, and the emission rate as a function of the length of the period of commitment. It is shown that the length of the period of commitment can have very significant effects on the equilibrium. We show numerically that at the initial date, as the length of commitment is increased, the potential gain from cooperation tends to diminish, increasing the disincentive to ratify the agreements. This suggests that considerable attention should be given to the determination of the length of such international agreements.