The adsorption and thermal reaction of H2S on the InP(100) surface is studied by synchrotron radiation (SR) soft X-ray photoelectron spectroscopy. In addition to molecular adsorption, H2S decomposes to form the dissociative species of S, HS, and H on the surface at 100K: The S atom of the sulfide species preferentially bonds to the In atom, and the H atom generated by the H2S dissociation bonds to the P atom. H2S molecules may physisorb on the surface in the form of an icelike multilayer/cluster at low temperatures, even at low coverages. The irradiation of SR white light can induce an alternative reaction of H2S with the InP surface to form a thicker sulfur layer than that obtained by thermal deposition. The resulting sulfur layer may provide chemical protection for the InP substrate from further reaction.