The effects of conditioning gas and pH on the collectorless flotation of chalcopyrite have been studied. To explain the changes in flotation performance during mineral oxidation, a study of the oxidation species produced in solution and on the mineral surface has been performed. Results of mineral dissolution, zeta potential measurements and X-ray photoelectron spectroscopy have shown that iron and, to a lesser extent, copper dissolve from the chalcopyrite lattice leaving a metal-deficient sulfur-rich surface and readsorb or precipitate as hydroxide species on the chalcopyrite surface. The hydrophobicity of the surface and therefore the chalcopyrite flotation are then controlled by these two processes, i.e. the metal dissolution that produces a hydrophobic surface and the metal hydroxide precipitation that produces a hydrophilic surface. The kinetics of formation and precipitation of these metal hydroxide species may control the surface hydrophobicity.