The impedance of the interface between an acidic electrolyte and polished electrodes of pyrite 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. At peak current densities the interface impedance is low and is dominated by activation controlled reactions involving pyrite dissolution and hydrogen gas evolution. The polarization curves have a series of step-like features at intermediate potentials due to current-limited reactions. These secondary reactions involve solid and/or aqueous reaction products from previous reactions. The high impedance portion of each reaction step corresponds to a limit current caused by either depletion of a particular solid reactant or employment of a current larger than can be carried by diffusion of aqueous reactants.Nonlinear behavior of pyrite was found to be independent of carrier type and conductivity. The potential of the anodic pyrite dissolution reaction was weakly dependent on pH. Potentials of cathodic reactions increased with increasing pH, indicating the involvement of H (super +) , as demonstrated by the evolution of hydrogen gas and H 2 S gas.