When the chemically synthesized poly(3-hexylthiophene) film was oxidized electrochemically in acetonitrile for 1 h, it showed three times a volume change after being transferred into chloroform solution to yield a permanent gel. The swelling increased as the potential was more positive. The absorbance at ca 302 nm, attributed to the n-->pi* transition in the conjugated chain, increased with the longer and more positive electrolysis, indicating development of the conjugation. In contrast, the absorbance at 765 nm, responsible for the electric conduction, decreased with the electrolysis. Both solubility of the polymer in chloroform and the electric conductance also decreased. The volume change was related quantitatively to the solubility with the help of Flory＇s theory of the swelling, in which the swelling ratio is proportional to the fifth power of the volume per crosslinked polymer molecule. FTIR spectra suggested that =C-H in the thiophene ring was more highly decomposed with application of the more positive potential to yield the crosslinking. The electrochemically synthesized film showed variations similar to those of the chemically synthesized film although the variations were less specific. The electrochemical gel was different from the gel initiated by benzoyl peroxide, which results in crosslinking between the hexyl chains.