A (NH4)(2)S-x-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 degrees 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 degrees C. A (2 x 1) structure appears for the treated surface annealed at 310 OC. The binding energy shift (-0.3 eV) of In 3d(5/2) and Sb 3d(3/2) is found for the (2 x 1) surface. The IPES spectra show that the density of states of unoccupied dangling bonds for surface indium is reduced by the (NH4)(2)S-x treatment. The binding energy shift and structure of the sulfide layer are discussed.