Chemical changes during effusion of substances in the binary ZnSe-ZnZn2Se4 system were observed during 21 studies by the simultaneous Knudsen- and torsion-effusion method. Principal vapor species were taken to be Zn(g), In2Se(g), and Se-2(g). Two intermediate solid phases were found in the temperature range 1100-1200 K, ZnIn2Se4(s) and (ZnSe)(5)In2Se3(s). The latter phase vaporized congruently throughout the temperature range, but it effused congruently only above 1150 K; it was not observed at room temperature. ZnIn2Se4(s) effused incongruently to a vapor rich in zinc sulfide and a condensed phase rich in indium selenide. Here, proposed binary pressure-composition phase diagrams of this system at approximately 1120 and 1190 K are given, and equations for all effusion reactions in the system are presented. Stability of the two intermediate condensed phases relative to the chemically binary constituents are small, Delta H degrees(298 K) = -12.5 +/- 3.5 kl/mol for ZnIn2Se4(s), and for the other phase it was less than the experimental error.