The fundamental-harmonic impedance of a two-step electrode reaction has been described. The dependence of the fundamental-harmonic impedance from the amplitude and frequency of the sinusoidal voltage perturbation has been determined. Simulated fundamental-harmonic impedance spectra have been determined of a two-step electrode reaction, based on the derived relations. The amplitude analysis of the high-frequency charge transfer resistance, the low-frequency charge transfer resistance and the intermediate product relaxation time allow the direct determination of the kinetic parameters of a two-step electrode reaction, corresponding to the zero value of the perturbation amplitude.