Charge transfer processes in poly-3-methylthiophene (PMT) and poly-3-octylthiophene (POT) film electrodes bathed with 0.05-0.5 M LiClO4 acetonitrile solutions were studied by cyclic voltammetry and electrochemical impedance spectroscopy. Semi-circle parameters R-ct and C-dl, Warburg constant sigma(w), effective diffusion coefficients D-eff, and low-frequency capacities C-LF were determined for PMT and POT films as functions of the bathing electrolyte concentration and the electrode potential. A comparison between the results of electrochemical studies of PMT and POT modified electrodes is performed in the paper. (C) 2004 Elsevier Ltd. All rights reserved.