The modification of clean GaAs(100) surfaces by in situ deposition of molecular sulfur was investigated by soft x-ray photoemission spectroscopy. Upon S treatment of the clean GaAs(100) sample at 435-455 degrees C in ultrahigh vacuum the formation of a three monolayer thick gallium sulfide-like compound is observed, which exhibits a (2x1) low-energy electron diffraction pattern. Due to the S modification on n-GaAs a reduction of the band bending by 0.35 eV is achieved, while the band bending on p-GaAs is increased by 0.17 eV, The subsequent Mg evaporation leads to the formation of a metal/semiconductor contact with a reacted magnesium sulfide-like compound at the interface. After 1 nm Mg deposition the Schottky barrier height of the S-modified Mg/n- GaAs(100) contact amounts to 0.44 eV, which is 0.18 eV lower than without. S modification, while the Mg/p-GaAs(100) Schottky contact exhibits an increase in the Schottky barrier height by 0.30 eV in comparison to the value of the unmodified Schottky contact (0.55 eV).