The copolymerization of aniline and thiophene with varying concentration ratios was studied in acetonitrile/tetrabutylammonium tetrafloroborate. In situ UV-vis and in situ surface resonance Raman spectroscopic studies revealed that polythiophene and polyaniline units of different lengths enter into the electropolymerization products. The copolymers deposited were either polyaniline-based or polythiophene-based polymeric structures depending on the initial concentration of the monomers used. (C) 2003 Wiley Periodicals, Inc.