The cathodic properties of the oxygen reduction at gas-diffusion electrodes were examined by an ac impedance analysis using an equivalent circuit in order to improve the stability of the gas-diffusion electrode in 30% concentrated alkaline solution at the temperature of 353 K. The effect of the wettability and penetration of the electrolyte into the gas-diffusion electrode was estimated by the impedance parameters of the charge-transfer resistance and the double-layer capacitance. Comparing the electrodes using Ag and Pt catalysts for different lifetimes, the Ag-loaded prototype electrode showed a higher performance than the Pt-loaded electrode. The cathode potential increased in a negative direction and the diameter of the Nyquist plots also increased for the longer used electrode. The surfaces of the gas-diffusion electrodes were observed and analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy. The thickness of the longer used electrode decreased because of the degradation. Carbon, polytetrafloroethylene, and silver were deformed by the damage of the electrode surface, and the peaks of sodium and oxygen were observed. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1938006] All rights reserved.