The interaction of atomically clean (100) and (111) crystallographic planes of FeS 2 with H 2 O vapor, O 2 , and a H 2 O/O 2 mixture was investigated. A combined high pressure/ultra-high vacuum (UHV) apparatus allowed the surfaces to be reacted at environmentally relevant pressures and studied with X-ray photoelectron spectroscopy (XPS) without exposure to the atmosphere. Neither surface exhibited significant reaction in pure O 2 . Exposure of FeS 2 (111) to H 2 O vapor resulted in significant oxidation, but under these same experimental conditions FeS 2 (100) exhibited a much smaller amount of oxidation. It is suspected that on FeS 2 (100), H 2 O only reacted on nonstoichiometric regions (i.e., defects). Both surfaces showed substantial reaction in H 2 O/O 2 . The amount of FeS 2 (100) and FeS 2 (111) oxidation in the H 2 O/O 2 mixture was more than simply the sum of the reaction observed individually in pure O 2 and H 2 O. This result suggests that there is a synergy between H 2 O and O 2 in oxidizing pyrite. In all cases, the amount of oxidation that occurred on FeS 2 (111) was greater than on FeS 2 (100). We believe that this experimental observation is due to a higher concentration of under-coordinated Fe in the outermost surface of FeS 2 (111), relative to FeS 2 (100).