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This paper presents results of a contact-angle study performed to revisiting the chemistry and kinetics of sphal-erite activation with Cu(II) on the light of the contact angle technique. Sphalerite (ZnS) activation is conducted with the aim of developing a cuprous sulfide layer with chemical affinity toward thiol-type collectors. This is achieved by manipu-lating the concentration of Cu(II), the conditioning time and the pH of the flotation slurry. The chemistry of the reaction consists of the reduction of Cu(II) to Cu(I) while copper replaces zinc from the external layers of the mineral crystal, to form a cupric sulfide (CuS) which spontaneously transforms to cuprous sulfide (Cu²S) and polysulfide (Sn²¯), rendering the activated sphalerite hydrophobic. The activation process follows a second-order kinetics in which the apparent control-ling step is the adsorption of Cu(II) on the unreacted mineral surface.