H-2 oxidation has been studied for Pt and Ni electrodes for different H-2/H2O ratios at 1000 degrees C in solid oxide fuel cells using yttria-tetragonal zirconia electrolyte by galvanostatic current interruption and electrochemical impedance spectroscopy. The results clearly indicate that the mechanism and kinetics of the H-2 oxidation reaction are strongly dependent on the catalytic activities of electrode materials, electronic conductivity of the electrolyte surface, and the water content in H-2 gas. The effect of water vapor in H-2 gas on the reaction kinetics is very much dependent on the electrode materials and is related to the partial pressure of oxygen. A reaction mechanism with two rate-limiting steps has been proposed and discussed.