The catalytic activities of Ni-mesh/YSZ samples for CH4 decomposition and reaction with reformed gases were compared under voltage-applied condition (fuel cell operation condition) by imaging analysis of labeled gases with secondary ion mass spectrometry (SIMS). The effect of applied voltages was compared in the mixture of CH4, D2O, and O-18(2) to label the movements of hydrogen and oxygen. A significant carbon deposition was observed at the Ni-mesh under zero-voltage condition. However, the applied voltage formed a thin oxide layer on the Ni surface and eliminated the deposited carbon on the Ni surface. Oxygen spill-over around the Ni/YSZ interfaces could effectively eliminate the deposited carbon. A possible reaction mechanism was considered for the optimum metal-oxide interfaces of SOFCs. (c) 2005 Elsevier B.V. All rights reserved.