The electron transfer between the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and the Fe(CN)(6)(3-/4-) redox couple in aqueous solution was investigated by electrochemical impedance spectroscopy (EIS). PEDOT was electrochemically deposited on platinum from aqueous solutions containing 0.01 M 3,4-ethylenedioxythiophene (EDOT) and 0.1 M poly(sodium 4-styrenesulfonate) (NaPSS) as supporting electrolyte. Pt/PEDOT(PSS) electrodes with polymer films of different thickness were investigated at different concentrations of the redox couple in 0.1 M KCl background electrolyte solution. Impedance spectra were obtained at the dc-potential corresponding to the formal redox potential of Fe(CN)(6)(3-/4-) (E-2 approximate to 220 mV) where the polymer is in the oxidized and electrically conducting state. The EIS data for the electrodes were fitted to an equivalent electrical circuit. The standard rate constant (k(o)) for electron-transfer between Pt/PEDOT(PSS) and Fe(CN)(6)(3-/4-) was determined by calculations based on the Butler-Volmer equation. The diffusion coefficient (D) of the redox couple was also calculated from the EIS data.