The electrochemical synthesis and redox response of a copolymer consisting of 3-octylthiophene and biphenyl units have been studied. The polymer material has been characterized by cyclic voltammetry, in situ UV–vis absorption spectroscopy, FTIR transmission and in situ FTIR attenuated total reflectance spectroscopy. Two different copolymer structures have been obtained when lithium hexafluoroarsenate and tetrabutylammonium tetrafluoroborate were used as electrolyte salts. The UV–vis absorption spectra and the IR bands of the neutral copolymer film are compared with the spectra of pure poly(para-phenylene) and poly(3-octylthiophene). The growth of infrared active vibrations are followed during oxidation of the copolymer film in monomer-free electrolyte solution and the spectral feature above 2000 cm−1 shows a reversible transformation of the film from the non-conducting to the conducting form when the copolymer is charged and discharged.