In order to elucidate the reaction zone in porous Ce0.9Gd 0.1 O1.95-δ anodes on yttria-stabilized zirconia (YSZ), ac impedance and steady-state polarization measurements are carried out in H2 - H2 O-Ar gas mixtures with different electrode thicknesses, 7, 20, and 45 μm 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 105 - 106 μF 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 Ce0.9Gd 0.1 O1.95-δ 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.