Catalytic CH4 decomposition and reactions of the reformed gases were examined on stripe-shaped Ni and Au electrodes on oxide substrates designed as optimum metal/oxide interfaces of SOFC anodes. The oxide substrates examined were Y2O3 -stabilized ZrO2 (YSZ) and Y2O3-doped CeO2 (YDC). The isotope labeling technique has been applied to visualize the distribution of reacted species, such as H, C, and O. In mixtures of CH4 + D2O + O-18(2), the samples were annealed at 973 K and SIMS analysis was carried out on the quenched samples. On the YSZ substrates, Ni was active for CH4 decomposition and carbon deposition, whereas the Au surface was inactive for the reaction. On the YDC substrates, the amounts of carbon deposited on the Ni surface were reduced due to their oxide ion conductivity and proton solubility. A possible reaction mechanism was considered for these metal/oxide interfaces. (C) 2004 Elsevier B.V. All rights reserved.