Scanning tunneling microscopy and low-energy electron diffraction have been used to study the adsorption and subsequent thermal desorption of molecular sulfur from the Si(001) surface. Room-temperature adsorption of sulfur resulted in the formation of an overlayer, displaying a high density of vacancies or defects, with the underlying Si(001) surface retaining the (2×1) reconstruction. Annealing this surface to 325 °C leads to the desorption of the sulfur overlayer and the appearance of coexisting c(4×4) and (2×1) surface reconstructions. Our data suggest that the c(4×4) reconstruction is an adsorbate-induced structure in which the sulfur creates defects during the desorption process. High-resolution filled- and empty-state images of the c(4×4) surface lead us to propose a missing-dimer defect model for this reconstruction.