The aqueous solution, in which an electrochemically oxidized poly(3-thiophene acetic acid) him was soaked, became acidic with an increase of the oxidation level of the film. This phenomenon was examined quantitatively through the relation between the electrode potential of the film in acetonitrile and the pH values of the water in which the him was transferred. Values of pK(a) of the film varied linearly with the applied potential less than 0.6 V. The results suggest that dissociation of the carboxyl group is activated by the oxidation of the thiophene ring. This effect was examined by the potential-dependence of FTIR bands of the methylene and the carboxyl group in poly(3-thiophene acetic acid) film. The absorbance of the carboxyl group showed a potential variation similar to the pK(a). The FTIR measurement was extended to the potential variation of the bands of the methylene group in poly(3-alkylthiophene) (the alkyls: methyl, hexyl, dodecyl and octadecyl). The absorbance of the C-K stretching vibration decreased, while the absorbance of the C-H wag and twist vibration increased with the increase in the oxidation level. The potential-dependence of these vibrational modes indicates that the oxidation at the thiophene ring stabilizes the vibrational energy at the 3-substituted alkyl chains. This stabilization is in accord with the pH variation in solution.