The kinetics of oxygen reduction on gas-diffusion electrodes has been extensively investigated, but quantitative data on the electrode characteristic or the mechanism of the electrode degradation are not sufficient. Therefore, electrode characteristics were evaluated by the change of the composition of the electrode, polytetrafluoroethylene content, or catalyst species of Ag or Pt to determine quantitatively the effect of the electrolyte penetration. In addition, a Nafion thin film or a plasma-polymerized film was coated on the surface of the electrodes to improve the stability of the electrode. The cathodic properties of oxygen reduction on the gas-diffusion electrode were examined by the ac impedance method. The effect of the penetration of the electrolyte into the gas-diffusion electrode was determined quantitatively by using the charge-transfer resistance and the double-layer capacitance, corresponding to catalyst consumption and wettability in the interafacial area of the electrode in highly concentrated alkaline solution at high temperature. From the acceleration test with ELAT (E-TEK Co.) electrodes, the charged thin film was found to minimize the penetration of alkaline solution into the electrode.