The oxidation of cobaltite, CoAsS, has been investigated by X-ray photoelectron spectroscopy. Products of the initial oxidation in air were deduced from surfaces abraded under liquid nitrogen rather than fracture surfaces, as fracture of the mineral sample studied took place preferentially along interfaces at which oxidized material was already present. Cobalt and arsenic oxides are formed in air under ambient conditions; however, sulfur is not involved in the initial oxidation. In air-saturated dilute acid, an oxidized layer is retained at the surface of cobaltite ore particles. Under similar aqueous conditions, pyrite, from which cobaltite often has to be separated, remains free of hydrophilic species.