ZnSe-GaAs heterostructures were synthesized by molecular beam epitaxy at room temperature on GaAs(1 1 0) substrates cleaved in situ, and at 290–320°C (Zn/Se beam pressure ratio = 1) on epitaxial GaAs(1 0 0) layers grown on GaAs(1 0 0) wafers. The epitaxial structures were characterized in situ by photoemission spectroscopy and reflection high energy electron diffraction. The photoemission-determined valence band offset was ΔE v = 1.10 ± 0.05 eV for ZnSe- GaAs(1 1 0) heterojunctions and ΔE v = 0.78−0.83 ± 0.07 eV for ZnSe-GaAs(1 0 0) heterojunctions. This germanium layers (4–6 monolayers thick) fabricated at the interface prior to ZnSe deposition yield a decrease in the overall valence band discontinuity for both ZnSe-Ge-GaAs(1 1 0) and ZnSe-Ge-GaAs(1 0 0) heterojunctions.