Charge transfer processes in poly-3-methylthiophene (PMT) and poly-3-octylthiophene (POT) film electrodes bathed with 0.05–0.5M LiClO4 acetonitrile solutions were studied by cyclic voltammetry and electrochemical impedance spectroscopy. Semi-circle parameters R ct and C dl, Warburg constant σ 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.