Electrochemical copolymerization of o-phenylenediamine (OPD) with o-methoxy aniline (OMA) in 1.0M HCl solution has been studied using cyclic voltammetric (CV) and in situ ultraviolet–visible (UV–vis) spectroelectrochemical techniques employing the indium tin oxide (ITO)-coated transparent glass as working electrode. The electrochemical features for the individual polymerizations of OPD and OMA are differently displayed in the copolymerization of OPD with OMA. The in situ UV–vis spectroelectrochemical results indicate that OPD and OMA are firstly oxidized to form the o-phenylenediamine cation radicals (OPDCR) and o-methoxy aniline cation radicals (OMACR) during the electrochemical copolymerization of OPD with OMA, then, mixed dimers/oligomers intermediates are formed by the cross-reaction between OPDCR and OMACR. The absorption peaks located at 403–422nm and 482–492nm in UV–vis spectra are assigned to these intermediates, which can also be confirmed by the results of CV and derivative cyclic voltabsorptogram (DCVA). The different voltammetric characteristics between the homopolymerization and copolymerization processes exhibit the occurrence of the copolymerization, and the differences between the copolymerization of OPD and OMA with different concentration ratios show the dependence of copolymerization on the concentration ratios of OPD and OMA in the feed. Furthermore, the copolymerization of OPD with OMA shows clear dependence on the applied potential. The copolymer has also been characterized by using Fourier transform infrared spectroscopy (FT-IR) and fluorescence spectroscopy. A plausible mechanism for the copolymerization of OPD with OMA has been suggested.