We have studied the formation of sulfide layers on the GaAs(100) c(8x2) gallium rich surface through thermal and photoinduced (193 nm) decomposition of H2S. H2S was adsorbed on the sample at 90-95 K. The sulfide layer desorbs as Ga2S beginning at 740 K. For thermal sulfide layer formation, it was found that more sulfur could be built up on the surface by repeated cycles of H2S exposure at low temperature followed by heating to 600 K. Several such cycles gave a sulfide layer saturating at about one monolayer. With photoinduced dissociation of H2S thicker sulfide layers were, produced, with no apparent saturation Limit after several layers had been formed. Auger data suggests that As remains’ below the gallium sulfide layer up to at least 600 K. Sulfide layers were found to inhibit oxidation of the substrate by NO2, e.g., sulfide coverages of 1.5 to 2 layers reduced the amount of oxide (Ga2O) formed by 70%-85%.