We have used the density functional theory (DFT) method with a plane wave-pseudopotential basis to compute the structure and properties of a model xanthate molecule (HOCS2-) and its adsorption characteristics on the pyrite FeS2(110) and (111) surfaces. Molecular calculations revealed that HOCS2- and CH3CH2OCS2- have similar head group electronic structure and, therefore, the use of the model xanthate is a justifiable approximation in simulations of xanthate head group attachment to FeS2 surfaces. Results from DFT calculations suggest that HOCS2- readily undergoes dissociation at the fourfold coordinated surface Fe on the (110) surface, and the bridging S of the (111) surface. These results suggest that xanthate may undergo chemisorption at defect sites on real FeS2 surfaces, which contain low-coordinate Fe sites and sites in proximity to cleaved S-S bonds. (C) 2003 American Institute of Physics.