X-ray photoelectron spectroscopic (XPS) studies revealed that the iron site on the chalcopyrite (CuFeS2) surface was preferably oxidized to the Cu site when exposed to an oxidizing environment. Extensive density functional theory calculations were performed to investigate the surface structure of chalcopyrite and its reaction with both molecular oxygen (O-2) and water. The adsorption and dissociation of a single O-2 molecule, a single H2O molecule, as well as both molecules at the Fe and Cu sites on the CuFeS2 (001) surface were studied. Consistent with our experimental observation, the Fe site was found to be preferred for the adsorption and dissociation of O2 due to its lower energy barrier and greater exothermicity. The dissociation of H2O on the CuFeS2 (001) surface by itself was found to be unfavorable both thermodynamically and kinetically. However, the surface formed upon O-2 dissociation was predicted to be much more reactive with H2O, which was attributed to favorable hydrogen transfer to the O site formed upon O-2 dissociation to hydrogen transfer to the S site due to the much weaker S-H bond than the O-H bond. (C) 2017 Elsevier B.V. All rights reserved.