The impedance of the interface between an acidic electrolyte and monomineralic, polished electrodes of galena, graphite, and chalcopyrite has been investigated at current densities in the nonlinear range (up to 5 mA/cm 2 ). The potential across a single interface relative to a reference electrode was measured in response to a current sinusoid of low frequency, 0.002 Hz. Polarization curves, or linear plots of current density versus electrode potential, consisted of distorted Lissajous patterns. Onset of a new electrochemical reaction resulted in a decrease in impedance of the interface, and hence increase in slope of the polarization curve.For some reactions, the electrical characteristics were diagnostic of bulk mineral composition. Diagnostic reactions include (1) mineral dissolution and gas evolution reactions at extreme anodic and cathodic potentials, (2) reactions at intermediate potentials which involve reaction products from previous reactions. Response is thus dependent on previous reactions and therefore on sample history. Anodic reactions were generally independent of pH, and consisted primarily of mineral dissolution reactions. Potentials of cathodic reactions increased with increasing pH indicating the involvement of H (super +) as demonstrated by the evolution of hydrogen gas and/or H 2 S gas.The potentials of the main graphite reactions were larger in magnitude than any of the sulfide reaction potentials. Measurements with polymineralic electrodes indicate that current flows mainly through minerals with reactions at less extreme potentials and consequently reactions involving other minerals at higher potential do not occur. Due to its more extreme reaction potentials, graphite does not respond in the presence of sulfide minerals. It appears that nonlinear phenomena could be used for mineral discrimination in drill hole logging.