The interaction of a natural face of FeS 2 (100), cleaned in ultra-high vacuum (UHV), with H 2 O and H 2 S has been investigated with X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and the photoemission of adsorbed xenon (PAX). PAX is sensitive to the short-range order of the surface and allows the effects of defects on the surface reactivity of FeS 2 (100) to be studied. PAX results suggest that both H 2 S and H 2 O bind most strongly to defect sites that we propose are, at least in part, sulfur anion vacancy sites. Whereas the majority of H 2 O adsorbate desorbs from these sites in the temperature interval of 200-300 K, H 2 S dissociates upon heating to 500 K into adsorbed surface hydrogen, S, and SH. This dissociation occurs on defect sites that then release part of the dissociation fragment, which is thought to be surface hydrogen, onto other regions of the pyrite surface that are proposed to be stoichiometric FeS 2 . Heating to 600 K causes further reaction of S containing dissociation fragments with sulfur-deficient sites to form new surface sites that resemble FeS 2 . The results also suggest that surface hydrogen dissolves into the pyrite bulk upon heating to 600 K.