Recently, much interest has been focused on the electrode reaction at the interface of porous Pt/stabilized zirconia in compound-gas mixtures such as CO-CO[sub 2], H[sub 2]-H[sub 2]O, CH[sub 4]-H[sub 2]O, and others, because this reaction is important as a model of the electrode reaction in zirconia cells either for sensors, fuel cells, water electrolyzer, or other applications. To elucidate the mechanism of the electrode reaction at the interface of H[sub 2]-H[sub 2]O porous Pt/stabilized zirconia, measurements were made on the electrode interface conductivity, [sigma][sub E], and the steady-state polarization current, I, as a function of H[sub 2] partial pressure, p[sub H2], and H[sub 2]O partial pressure, p[sub H2O], up to 800 C. The rate of the electrode reaction at 700 to 800 C in H[sub 2]-H[sub 2]O atmospheres is faster than that in CO-CO[sub 2] and slower than that in O[sub 2] 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.