The corrosion inhibiting and accelerating effect of l-cysteine on Zn were correlated with the formation of Zn oxides by means of a novel coupling of impedance spectroscopy (EIS) with atomic emission spectroelectrochemistry (AESEC). A plateau of anodic Zn dissolution was discovered in the cathodic branch of the polarization curve, hidden by the large cathodic current. The effect of l-cysteine at all pH values and all concentrations was to increase the plateau current. The effect was most pronounced at high pH suggesting a synergy between hydroxide and l-cysteine. Due to the presence of Zn plateau current, Tafel extrapolation resulted in a wrong estimation of corrosion currents. The contribution of the anodic Zn oxidation in the plateau region to the total current was inversely proportional to the frequency of the AC perturbation.