Modern surface science techniques have been used to investigate the chemisorption and reaction of H2O on the (100) face of pyrite (i.e., FeS2). The surface of pyrite under investigation is prepared by bombarding an "as-grown" surface (1 cm2) of FeS2(100) with low energy He+. We believe that this preparation method produces a surface that is representative of the naturally occurring surface. Both low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy have been used to characterize the pyrite surface. LEED suggests that the surface is unreconstructed. High resolution electron energy loss spectroscopy (EELS) has been used to obtain the vibrational spectrum of water adsorbed on pyrite and after its thermally induced decomposition. EELS shows that a fraction of the water monolayer dissociates below room temperature leading to the appearance of vibrational bands in the EELS spectrum that are tentatively assigned to iron oxide. Temperature programmed desorption studies show that H2S desorbs from the surface during this oxide formation. Mechanistic details of these surface reactions will be presented.