For the evaluation of electrode reaction at H-2-H2O(g), Pt/BaCe0.95Y0.05O3-delta, the electrode impedance and steady-state polarization measurements were examined as the functions of hydrogen partial pressure. P-H2, steam partial pressure, P-H2O and hydrogen activity on the electrolyte surface at triple phase boundary (TPB), a(H), using three-terminal method at 773 and 873 K. The electrode interfacial conductivity strongly depended on P-H2. From the results of the polarization measurements, the a(H), dependence of the reaction rate can be related to the contribution of adsorbed hydrogen atoms on the BCY surface at TPB. Additionally, the P-H2 dependence of the reaction rate is attributed to the contribution of the adsorbed hydrogen atoms on the Pt surface. In order to explain the P-H2 dependence of reaction rate, we propose the two electrode reaction models by the magnitude of the concentration of adsorbed hydrogen atoms on the Pt surface. The following reaction processes were considered as the rate determining reaction step in H-2-H2O(g). Pt/BaCe0.95Y0.05O3-delta: (i) the surface diffusion of H-ad (Pt) near TPB, and (ii) the dissociative adsorption of H-2 on the Pt surface at TPB. The reactions (i) and (ii) take place in parallel. (C) 2009 Elsevier B.V. All rights reserved.