Sphalerite is commonly floated in an alkaline environment and under such conditions the overall activation mechanism is obscured by the hydrolysis of Cu2+ and precipitation of Cu(OH)(2). Electrokinetic and kinetic flotation tests showed that the activation of sphalerite is a two-stage process; the activation products formed on the surface of sphalerite in near neutral and in alkaline solutions in the first rapid activation stage are ＇＇flotation-inactive＇＇. These layers, when formed in an alkaline solution, are unstable and are transformed into the ＇＇flotation-active＇＇ form in the second stage. However, the surface layers formed during activation in alkaline environment by copper in the presence of lead ions are very stable and even after a long conditioning time the flotation rate is not improved. Consequently, while both Cu2+ and Pb2+ activate sphalerite flotation, quite unpredictable effects may result when both are present in the solution.