We have studied via LEED, AES, and EELS the (110) and (100) interfaces between GaAs and ZnSe or Se deposited in UHV on sputter-annealed substrates. These interfaces are reactives. Although nearly stoichiometric (ϑ>20 Å) crystalline ZnSe films can be grown on GaAs by congruent evaporation, the ZnSe–GaAs interfaces are nonstoichiometric and include a thin intermediate (Ga, Se) compound layer due to a Se–As exchange taking place during the early stages of growth. Various phases of the (Ga, Se) compound can be obtained, depending on the availability of Se at the interface. The structure and chemical properties of the Se–GaAs interfaces are also investigated and related to the chemical reaction observed during the formation of the ZnSe–GaAs interfaces.