In order to elucidate the reaction zone in porous Ce0.9Gd0.1O1.95-delta anodes on yttria-stabilized zirconia (YSZ), ac impedance and steady-state polarization measurements are carried out in H-2-H2O-Ar gas mixtures with different electrode thicknesses, 7, 20, and 45 mu m anodes. Steady-state polarization current becomes larger as the electrode becomes thicker, while the current per unit surface area shows similar value. Therefore, the current seems to be a function of the electrode surface area. In ac impedance measurements, an extremely large pseudo-capacitance is observed to be as large as 10(5)-10(6) mu F cm(-2). The measured pseudo-capacitance is caused by the nonstoichiometric perturbation of the oxygen content in the anode material. Both steady-state polarization and ac impedance measurements suggest that the reaction zone extends from the interface of YSZ/porous Ce0.9Gd0.1O1.95-delta anodes toward the outer gas phase. The distribution of electrochemically active zone is semiquantitatively estimated. The activity for the surface reaction gradually decreases with increasing the distance from the electrode/electrolyte interface, and the reduction rate of the activity becomes high as the electrode thickness increases. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2975322] All rights reserved.