The effect of the redox hysteresis of poly(3,4-ethylenedioxythiophene) films on the electrochemical mechanisms analyzed by impedance has been studied. Impedance spectra were recorded at fixed steady-state potentials under both negative and positive sweeps. The effects of film thickness were also investigated by varying the polymerization charge used. It has been shown that not only do quasi-equilibrium circuit elements (low-frequency capacitance) vary with the potential sweep direction but also kinetics elements (interfacial charge transfer resistance) depend on it, both exhibiting hysteretic effects. The onset of the charge transfer resistance increase is shifted approximately -300 mV during the negative polarization. (c) 2004 Elsevier B.V. All rights reserved.