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Linear sweep voltammetry with a freshly renewed surface of a chalcopyrite false(CuFeS2false) mineral electrode shows a small anodic prewave in acid electrolytes. The main anodic reaction, with complete dissolution of copper and iron, occurs at more positive potentials. Experiments under nitrogen show that the prewave is not a consequence of atmospheric oxidation during surface preparation. The prewave represents a surface oxidation of chalcopyrite to form a thin passivating film. The charge passed in forming this film is around 2 mC cm−2 for a polished surface. The prewave reaction is identified with the initial phase of the oxidative dissolution or leaching of chalcopyrite. Contrary to the reported behavior at 80°C, the surface film is stable at 25°C for long periods at open circuit. Various evidence, including the relative charges passed in forming and then reducing the surface film, and the presence of dissolved copper, leads to the following suggested stoichiometry for the prewave reactionCuFeS2→0.75normalCuS+0.25Cu2++Fe2++1.25S+2.5e−The product layer containing normalCuS and S is about 3 nm thick.