AbstractThis work develops the theoretical equations for a single potential step—open circuit relaxation chronoabsorptometry for use in cases when the other species also displays absorption at the wavelength being observed for oxidation. A data analysis method is proposed to determine the rate constant of the catalytic reaction. The rate constants of ferrocyanide—aniline and ferrocyanide—antipyrine systems are determined by an optically transparent spectroelectrochemical method. The result coincides with the value determined by the electrochemical method.