Results of in situ UV-visible (UV/vis) spectroelectrochemical studies of the electro-oxidation of o-phenylenediamine (OPD), o-toluidine (OT), and their mixtures, as well as the redox activity of the obtained polymer films, performed in an aqueous acidic solution at an indium-doped tin oxide-coated glass electrode, are reported. UV-vis spectra recorded at different time intervals during the electrolysis of mixed solutions, containing a constant concentration of OT and various OPD concentrations in the feed, are different from those obtained with single monomer solutions. A band at lambda=497 nm was assigned to the head-to-tail mixed dimer/oligomer resulting from the cross reaction between OPD and OT cation radicals. At lower OPD feed concentration this band appears as the major band. The dependence of the spectral characteristics of the copolymer films on the OPD feed concentration was deduced. The UV-vis spectra of the copolymers reveal that the copolymer backbone probably consists of a mixture of copolymer chains with different monomer contents and has a significant number of block segments. Fourier transform infrared spectral analysis of the copolymers suggests the presence of both OPD and OT units and thus the formation of a copolymer during the electrolysis of mixed solutions of OPD and OT.