Theoretical investigation of potentiostatic electrolysis of a metallic salt in three component electrolyte solution was carried out for a cell consisting of two identical parallel electrodes. Analytical and numerical results are given for polarization curves for electrochemical cells with arbitrary values of transfer coefficient alpha and exchange current density. Theoretical analysis of the electrodiffusion problem, based on an exact solution of the Nernst-Planck equations with boundary conditions of Butler-Volmer type, led to a formula for the polarization curve that is similar to the Tafel equation but with an effective transfer coefficient alpha(eff) = alpha(1 - alpha). It was shown that, under certain conditions, the polarization curve can have two inflection points.