The theory of square-wave voltammetry of three-electron electrode reaction that consists of one-electron transfer which is followed by the simultaneous transfer of two electrons is developed. If the first step is irreversible and the second one is quasireversible, the extremes of components may be widely separated, resulting in the net response with two peaks. This phenomenon could appear and disappear as the frequency is increased. It is proposed that the particular frequency is measured, at which the two peak currents of the net response are identical. For this critical frequency and the standard signal parameters, the relationship between the standard rate constant of the first step, the transfer coefficient of this step and the difference between standard potentials of two steps is calculated. (C) 2011 Elsevier Ltd. All rights reserved.