To elucidate the mechanism of the electrode reaction at the interface of H-2-H2O porous Pt/stabilized zirconia, measurements were made on the electrode interface conductivity, sigma(E), and the steady-state polarization current, I, as a function of H-2 partial pressure, p(H-2), and H2O partial pressure, p(H2O), up to 800-degrees-C. The rate of the electrode reaction at 700 to 800-degrees-C in H-2-H2O atmospheres is faster than that in CO-CO2 and slower than that in O2 rich atmospheres by 1 to 2 orders of magnitude. The rate-determining reaction process was the exchange of adsorbed OH radicals between the Pt- and the stabilized zirconia-surface at the triple-phase boundary (TPB) of gas/Pt/stabilized zirconia. When the oxygen activity, a0, (vs. O2 gas of 1.013 x 10(5) Pa) on the zirconia surface at the TPB was higher than 10(-10), the predominant rate-determining reaction and the rate equation were given by H(ad) (Pt) + OH(ad) (SZ) --> H2O(ad) (Pt) + V(ad) (SZ) and I = k1theta(H)(Pt)1/2theta(OH)(SZ)1/2 - k1’theta(H2O) (Pt)1/2theta(v) (SZ)1/2, respectively. Those in the region of a0 < 10(-12) were expressed by H(ad) (Pt) + H2O(ad) (SZ) --> H2O(ad) (Pt) + H(ad) (SZ) and I = k2theta(H) (Pt)1/2theta(H2O) (SZ)1/2 - k2’theta(H2O)(Pt)1/2theta(H) (SZ)1/2, respectively. Here, k1, k1’, k2, and k2’, are the rate constants, and theta denotes the coverage of the adsorbed species indicated by suffix on the surface of the solid shown in parentheses.