Simultaneous in situ measurements by both ESR and UV-vis spectroscopy have been carried out at the same electrode for the first time during a single electrochemical experiment in order to spectroscopically characterize the electrochemical reaction products. The experimental technique, including a special TE(102) optical ESR cavity and an electrochemical cell for both ESR and UV-vis spectroscopy in transmission, is described. It is shown that the UV-vis absorbance measured in situ, as well as the ESR intensity characteristics of the electrochemical system under study, can be understood in terms of the faradaic current. The reliability of this system is proved by measuring the organic redox couple of methyl-substituted p-phenylenediamine and by comparing calculated and experimental curves.