The surface oxidation of synthetic and natural samples of enargite and tennantite has been monitored by X-ray photoelectron spectroscopy, XPS. The minerals were conditioned at pH 11.0 in an aqueous solution purged with nitrogen gas for 20 min or with oxygen gas for 60 min. The XPS results show that the oxidation layer on the mineral surface is thin. The surface oxidation products comprise copper and arsenic oxide/hydroxide, sulfite, and a sulfur-rich layer made of metal-deficient sulfide and/or polysulfide. The proportion of all of these oxidation products at the mineral surface is more important when the minerals are treated in more oxidizing conditions (i.e., with oxygen gas and for a longer time) for tennantite than for enargite and for the natural samples than for the synthetic samples. Different arsenic sulfide species have been found at the surfaces of enargite and tennantite: As4S4 Or As2S3 constitutes the major arsenic sulfide species at the surface of enargite, but these are the minor arsenic sulfide species at the surface of tennantite and in the bulk of both minerals. This difference is not related to a surface impurity in the natural enargite sample as it is also observed in the synthetic enargite sample.