We report synchrotron radiation, soft-x-ray photoemission spectroscopy studies of ZnSe-GaAs heterojunctions fabricated by molecular beam epitaxy in situ on GaAs(001)2X4 substrates, Measurements of the band offsets confirm that interfaces grown in Se-rich conditions exhibit relatively low valence band offsets (as low as 0.5 eV), while interfaces grown in Zn-rich conditions show relatively high valence band offsets (as high as 1.3 eV). In the Se-rich case, the improved surface sensitivity of the technique revealed previously unreported contributions to the As 3d and Ga 3d core lineshapes, with substantial (0.8-2 eV) chemical shifts. The shifts, as well as the coverage and escape-depth dependence of the results suggest enhanced atomic intermixing across Se-rich interfaces, with the formation of both Se-As and Se-Ga chemical bonds.