Active parts for CH4 decomposition and electrochemical oxidation of reformed gases were investigated at the well defined Ni-mesh/oxide interfaces in mu m level. The effective reaction areas were determined by isotope labeling technique under the mixture of CH4, D2O, and O-18(2) at 1073 K. A deposition of carbon preferentially occurred on the Ni-mesh surface on Y2O3-stabilized ZrO2 (YSZ) and SM2O3-doped CeO2 (SDC) electrolyte oxides. A slight reduction of carbon deposition was observed on SDC substrate under non-polarized condition. The electronic and structural properties changes of Ni were observed by XANES/EXAFS analysis. By anodic polarization, a significant reduction of deposited carbon was observed on Ni-mesh surface in Ni-mesh/YSZ and Ni-mesh/SDC samples. Oxygen spill over can be effective for eliminating the deposited carbon on the Ni-mesh. Hydrogen and isotope oxygen concentration (O-18(2)) on the Ni-mesh was changed by the oxide substrates under anodic polarization. (c) 2006 Elsevier B.V. All rights reserved.