The relation between electrode microstructure and gas diffusion has been investigated with different morphologies of Pt electrodes by using AC impedance techniques. The measurements were carried out at temperatures of 873-1273 K and oxygen partial pressure (P-O2) of 0.01-1 atm.Gas-phase diffusion was observed only for high-performance electrodes at the high-temperature (1073-1273 K) and low-oxygen-partial-pressure regions (<0.1 atm P-O2). Considering the physical and electrochemical characteristics of impedance arcs, it was found that the arc at the frequency of below 1 Hz was related to gas conversion resistance, while the arc at the frequency of around 10 Hz represented pore diffusion resistance through the current-collecting part. For a thick electrode with a low porosity, however, gas diffusion resistance through pores of an electrode was observed at a frequency of around 100 Hz.From the results of a comparison of electrode performances with different electrode micro structures, electrochemical reaction sites (ERS) are supposed to be located at the peripheral line of Pt and YSZ as well as the Pt/YSZ interfaces where reaction gas can easily diffuse. (C) 2003 Elsevier B.V. All rights reserved.