In situ reflectance-difference studies of initial stages of ZnSe growth on GaAs(001) surfaces are reported. ZnSe layers with thicknesses less than 4 nm were grown by molecular beam epitaxy in both layer-by-layer and island-growth modes. It is found that the reflectance-difference spectra for this thickness range are clearly dependent on ZnSe thickness as well as on the growth mode, This observation indicates that the surface electronic structure develops with thickness and in a manner sensitive to the mid-or long-range order of the surface. The interface-induced peak at 2.8 eV, intensity of which is correlated with the extent of the interfacial Ga-Se bond formation, is pinned during the course of growth, which indicates that atomic rearrangement or mixing at the interface is minimal once several monolayers of ZnSe are deposited on GaAs.