Poly(3-methylthiophene) (P3MT)-based Schottky barrier diodes were prepared and their capacitance and conductance were measured as a function of frequency. The loss tangent of these structures shows a maximum which depends on the temperature according to an Arrhenius law. An average activation energy of 0.3 eV is deduced from the position of this maximum, A similar value was found from the temperature dependence of the I-V characteristics of these diodes. In addition, the short-circuit photocurrent versus the photon energy reveals a peak at 1.66 eV which is attributed to the anti-bonding level of the polaron. (C) 2002 Elsevier Science B.V. All rights reserved.