In order to unravel the oxidation mechanism(s) of chalcopyrite (CuFeS2) surfaces, the surface anisotropy of CuFeS2 was investigated under both dry and aqueous atmospheres through a fundamental density functional theory (DFT) study. Results showed that the CuFeS2 (112) surface was the preferred cleavage surface, rather than the commonly-assumed (001) surface. Iron oxides were formed when contacting with either molecular O-2 or dissociated O atoms, but sulfoxy species were only formed in the presence of dissociated O atoms, exposing Cu atoms on the S-terminated (001) and (112) surfaces. This indicates that strong oxidation conditions are beneficial to CuFeS2 oxidation and the release of metals into solution. In addition, CuFeS(2 )was preferentially oxidized by O-2 prior to the adsorption of H2O under an aqueous condition with both H2O and O-2. The H2O adsorbed on the O-oxidized surface further promoted CuFeS2 oxidation, resulting in the formation of a hydrophilic surface, rather than the naturally hydrophobic surface. This study has unveiled, for the first time, the mechanisms of the oxidation of the most stable and reactive CuFeS2 (112) and (001) surfaces under both dry and aqueous environments at an atomic level, with potential applications in CuFeS2 flotation and leaching processes.