The influence of electrochemical system non-linearity on impedance measurement is considered for a two-step reaction with an intermediate adsorbed species. Theoretical expressions for the non-linear polarization resistance and the non-linear charge transfer resistance are derived corresponding, respectively, to low frequency and high frequency sinusoidal modulation of the electrode potential. A formal integration procedure is used to obtain the electrochemical response to the sinusoidal perturbation, whatever the input signal frequency, and, from the fundamental component of the response, to derive the non-linear admittance and impedance expressions. Tests are carried out to check the validity of the theoretical results, firstly using the limiting low frequency and high frequency expressions and finally solving numerically the kinetic equations. The influence of the perturbation amplitude on the impedance diagram is studied. The theoretical predictions invalidate previous derivations found in the literature.