A study of the adsorption of sulfur on the GaAs(100) surface after in situ thermal desorption of a protective As capping layer is presented. The sulfur flux was generated by the decomposition of silver sulfide in an UHV-compatible electrochemical cell. Use of As-capped samples provided a means to study the interaction of sulfur with both the c(2×8) and (4×1) surface reconstructions. Scanning-tunneling-microscopy images of the sulfur-covered surface indicated the formation of disordered surface layers which display a diffuse (1×1) low-energy-electron-diffraction pattern. This (1×1) phase is attributed to the symmetry of the bulk structure visible through the disordered surface overlayer, caused by the adsorbed sulfur breaking the surface dimer bonds. Synchrotron-radiation core-level photoemission spectra indicate evidence of sulfur bonding to both gallium and arsenic at room temperature, but that the relative magnitude of these bonding interactions depends on the Ga/As ratio of the clean surface. Sulfur 2p photoemission spectra from the annealed surfaces show that sulfur diffuses into the topmost atomic layers as well as bonding to the surface. Annealing the sulfur-covered surface above 400 °C results in the formation of a (2×1) low-energy-electron-diffraction pattern with a dimer-row structure clearly visible in scanning-tunneling-microscopy images. Our results would suggest that the adsorption of sulfur on the c(2×8) clean surface results in dimer rows consisting of both arsenic and sulfur dimers, while only sulfur dimers are observed after adsorption on the (4×1) surface. The degree to which the clean surface band bending is altered on these respective surfaces appears to be related to the precise chemical composition of the dimer rows.