Oxygen reduction to hydroperoxide ion in a 0.5 mm-thick porous uncatalyzed carbon-polytetrafluoroethylene electrode has been studied by electrochemical impedance spectroscopy using different experimental conditions such as KOH concentrations from 1.0 to 6.0 M, constant applied potentials with steady cathodic current densities in the range 5-100 mA cm(-2), O-2 partial pressures of 0.21-1.0 atm and temperatures from 5.0 to 55.0 degreesC. The Nyquist diagrams show a main semicircle, assigned to dominating charge transfer and weak adsorption, together with a low-frequency semicircle, assigned to diffusion. The parameters of the equivalent circuit have been related to structural aspects of the electrode. The fitting results indicate increasing an electroactive area towards the electrolyte side when the overpotential increases. The low-frequency behaviour, interpreted using the finite-length diffusion model, is ascribed to the diffusion of electrogenerated hydroperoxide ion through the electrode pores. The gas diffusion cathode apparently has higher electroactive area approaching the electrolyte side and also, some tortuous electrolyte channels penetrating some tens of microns into its porous structure. The increase in its double layer capacity with increasing KOH concentration, cathodic current density and temperature, is explained by a higher electroactive area due to an increase in the electrode wetting. (C) 2003 Elsevier Science B.V. All rights reserved.