A vapor-deposited GaS passivating layer is formed on GaAs(100) with alpha-Ga2S3 powder used as a single-source precursor for the deposition. The films grown show near-single-crystal quality, and are characterized by Auger electron spectroscopy and x-ray diffraction spectroscopy. The band-edge discontinuities of the GaS/GaAs heterojunction are determined to be 1.9 eV for the valence band and 0.3 eV for the conduction band, respectively, by ultraviolet photoelectron spectroscopy and electron-energy-loss spectroscopy. It is also observed that the valence-band structure of the GaS overlayer becomes much sharper after annealing.