A (NH4)2Sx-treated InSb(001) surface has been studied by using x-ray photoelectron spectroscopy, low-energy electron diffraction, and inverse photoemission spectroscopy (IPES). A thick sulfide layer is formed on the as-treated and annealed surfaces at less than about 400 °C. The thickness of the sulfide layer is estimated to be about 6–7 ML. Sulfur is bonded to both In and Sb in the as-treated surface layer although it is bonded only to indium in the layer annealed at more than 310 °C. A (2×1) structure appears for the treated surface annealed at 310 °C. The binding energy shift (-0.3 eV) of In 3d5/2 and Sb 3d3/2 is found for the (2×1) surface. The IPES spectra show that the density of states of unoccupied dangling bonds for surface indium is reduced by the (NH4)2Sx treatment. The binding energy shift and structure of the sulfide layer are discussed. © 1999 American Vacuum Society.