Electrochemically synthesized films of poly(3-methyl thiophene) have been oxidized and reduced at controlled potentials in an acetonitrile solution of tetramethylammonium hexafluorophosphate. The polymer films thus treated are found to exhibit p- and n-type semiconductance based on the measurements of absorption spectra, work functions and conductivities together with the current-voltage measurements with and without illumination for the polymer films sandwiched between dissimilar metals.