This note determines a rule to share a surplus gained when two countries or
regions agree to coordinate their policies to reduce downstream pollution.
An intertemporal decomposition scheme for the total side payment is propos
ed. This scheme has the following individual rationality property: in each
subgame that starts along the cooperative trajectory, one country is guaran
teed to receive a higher payoff in the cooperative solution than in the dis
agreement solution. For this country another notion of individual rationali
ty obtains: this country will at any instant of time during the play of the
game receive a higher payoff in the cooperative solution than in the disag
reement solution. (C) 2001 Elsevier Science B.V. All rights reserved.