The electron transfer between the ferri/ferrocyanide redox couple and poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes has been investigated by electrochemical impedance spectroscopy (EIS). Different thicknesses of the conducting polymer films were investigated (i) at a constant concentration of ferrocyanide in the solution at different applied de potentials and (ii) at the open circuit potential with different ratios of Fe(CN)(6)(3-) :Fe(CN)(6)(4-) in the solution. PEDOT was prepared by galvanostatic electropolymerization on platinum electrodes from aqueous solutions containing 0.01 M 3,4-ethylenedioxythiophene (EDOT) and 0.1 M poly(sodium 4-styrenesulfonate) (NaPSS) as the supporting electrolyte. All impedance spectra were obtained in aqueous solutions with 0.1 M KCl as the supporting electrolyte at dc potentials, where the polymer is in the oxidized state. The EIS data were fitted to an equivalent electrical circuit resembling the Randles' circuit, where the double layer capacitance is replaced by the bulk redox capacitance and the associated transport impedance of the conducting polymer. The same potential and thickness dependence of the (R-ct) were obtained both (i) by varying the concentration ratio of the redox species in the solution and (ii) charge transfer resistance R-ct by applying different potentials at a constant concentration of ferrocyanide. The potential dependence of k(0) calculated from Rct indicates that the conducting polymer influences the rate of electron transfer for the ferri/ferrocyanide redox couple. (C) 2003 Elsevier B.V. All rights reserved.