The copolymerization of o-aminophenol（OAP） and o-phenylenediamine（OPD） was studied by using in situ ultraviolet-visible（UV-Vis） spectroelectrochemical and cyclic voltammetric techniques.The different voltammetric characteristics in homopolymerization and copolymerization processes exhibited the occurrence of the copolymerization between OAP and OPD in 1 mol/L HCl solution.The in situ UV-Vis spectra showed that,during the copolymerization of OAP and OPD,OAP and OPD were firstly oxidized to generate their cation radicals,then the new-formed dimers/oligomers intermediates:phenazine type structure and PANI like backbone structure were formed by the cross-reaction of the cation radicals of OAP and OPD with the OAP and OPD monomers or their cation radicals in solution.The dimers/oligomers intermediates continued to react to form copolymer，showing two absorption peaks located at 477 nm and 419 nm,respectively.Fourier transform infrared Spectroscopy（FT-IR） was used to characterize the formation of the copolymer of OAP and OPD.Further studies showed the difference between the copolymerization of OAP and OPD with different concentration ratios,indicating the dependence of the copolymerization on the concentrations of OAP and OPD.