Due to the nonlinear nature of a corroding system, a sinusoidal potential distortion results in an ac current response, which contains higher harmonics. These harmonics can be measured and evaluated to obtain kinetic information, for example the corrosion rate and Tafel parameters. A model was developed for the case that the anodic reaction was charge transfer controlled and the cathodic reaction was under mixed control, i.e., both the charge-transfer and diffusion determine the reaction rate. This model was used to obtain the corrosion rate and Tafel parameters for the corrosion of mild steel in an aerated sodium sulfate solution.