The degradation behavior of the sulfur-treated InP surface at relatively low temperature has been investigated with x-ray photoelectron and photoluminescence (PL) spectroscopy. The results showed that the treated surfaces were oxidized toIn2O3,InPO3, andInPO4at 250 °C and in a vacuum of10−3Torr for 20 min. As the holding time for S-treated InP under a vacuum of10−3Torr increased, the PL peak caused by the band edge transition decreased without the formation of oxides. It was therefore suggested that the decrease of the PL intensity for S-treated InP is only related to the generation of phosphorous vacancies at the surface, not to oxide formation. The usefulness of a thin S overlayer on III–V semiconductors was also discussed. ©1997 American Institute of Physics.