The effects of electrochemical system non-linearity on low frequency impedance measurements are investigated and the influence of the electrolyte resistance on such measurements is considered under non-linear conditions. Theoretical expressions are derived that relate the polarization resistance measurement error, due to non-linearity, to the successive derivatives of the current vs potential characteristic when using a frequency response analyzer and a sinusoidal modulation of the potential or the current as the input signal. As an example, the exponential Tafel law is investigated theoretically and a semiconductor diode, which can be considered as a model system for exponential non-linearity, is studied experimentally to test the theory. Theoretical and experimental results invalidate a previous derivation in the literature.