Following the theory used to study the semiconductor/electrolyte interface the differential capacitance of poly(3-methylthiophene) films has been determined from measurements with a lock-in amplifier and by electrochemical impedance spectroscopy (EIS). According to our findings, the best results were obtained by EIS because the space charge capacitance can be separated from the other capacitances. Using Mott-Schottky plots (C-2 vs. E) we obtained the flat band potential E-fb = 80 mV and the carrier density N = 6 x 10(17) cm(-3) for the PMeT film in contact with the electrolyte, where dissolved O-2 played the role of the electron acceptor. The determined width of the depletion layer is 0.04 mu m. We also investigated the photoelectrochemical response of the PMeT him. The plot of the square of the photocurrent vs. potential yields E-fb = 90 mV, in good agreement with the EIS measurement. The dependence of the photocurrent with the frequency of the incident light shows that PMeT has a long response time (order of ms), compared to an inorganic semiconductor. The band gap was also determined from the photocurrent spectra. The value obtained, for a direct transition is 1.9 eV and is coincident with the value obtained from the ab:sorption spectra. (C) 2000 Elsevier Science B.V. All rights reserved.