We implement a family of efficient proposals to share benefits generated in environments with externalities. These proposals extend the Shapley value to games with externalities and are parametrized through the method by which the externalities are averaged. We construct two slightly different mechanisms: one for environments with negative externalities and the other for positive externalities. We show that the subgame perfect equilibrium outcomes of these mechanisms coincide with the sharing proposals.The authors gratefully acknowledge the financial support from BEC 2003-01132. Macho-Stadler and Pérez-Castrillo also acknowledge the finantial support of Generalitat de Catalunya (2001 SGR-00162 and Barcelona Economics, CREA).