X-ray photoelectron spectroscopy (XPS) was used to study the kinetics and mechanisms of sphalerite activation in a 10(-4) M CuSO4 solution at pH 9.2. The activation was fast during the first 10 min, after which the rate decreased exponentially. The increase in the amount of copper ion uptake was accompanied by the displacement of zinc ions from the sphalerite surface, indicating that the mechanism of copper uptake is one of displacement rather than adsorption. The XPS data show that the kinetics of activation increased considerably in a deoxygenated CuSO4 solution. Both the conventional XPS and synchrotron radiation XPS (SR-XPS) analyses show that copper activation involves a mechanism in which Cu2+ ions are reduced to the Cu(I) state, while the sulfur in ZnS is oxidized. Activation in the absence of oxygen and at an open circuit results in the formation of a CuS-like product, while in an air-saturated solution copper polysulfides are formed. The latter mechanisms are supported by the appearance of two distinct S(2p) doublets representing S- and S-0 species at higher binding energies. When copper-activated sphalerite was conditioned in an air-saturated solution of pH 9.2, the CuS-like activation product is oxidized in preference to unactivated ZnS. The oxidation resulted in a loss of copper from the sphalerite surface, which may be detrimental to flotation.