The tunability of band offsets in ZnSe/GaAs(001) heterovalent heterostructures is investigated. The interface composition, Ga/As, is controlled by means of Zn or Se treatment or by thermal etching of the GaAs surfaces before the growth of ZnSe. Consequently, it is revealed by X-ray photoemission spectroscopy that artificial control of Ga/As from 1.0 to 2.8 leads to variation of the valence band offsets from 0.6 to 1.1 eV. Based on the electron counting model and the layer-attenuation model, a structural model which is responsible for the Ga-rich interface and for the increase of valence band offset is proposed, in which the As plane just below the interface consists of As, anti-site Ga and As vacancies. The electronic properties of then-ZnSe/p+-GaAs heterojunction diodes (HDs) provide further evidence of the tunability of the band offsets at the interface, that is, the diffusion potentials in the HDs are modified according to the interface compositions. ©1997 American Institute of Physics.