A new vacuum-compatible passivation technique for III-V compound semiconductors has been developed. Sulfur passivation of GaAs(100) is produced by ultraviolet photolytic deposition of a sulfide species from vapor phase elemental sulfur. Photoluminescence studies of the photosulfided GaAs reveal a degree of passivation greater than or equal to that produced by conventional (NH4)2S solution treatments. X-ray photoelectron spectroscopy has shown that the sulfur resides on the surface as a single reduced sulfur species, either as sulfide or disulfide, indicating complete fragmentation of the S8 ring by UV light in proximity to the surface. The degree of photosulfidation depends strongly on surface preparation as demonstrated by the described surface oxide removal studies.