Influence of the materials and cell configuration, especially anode thickness, on the performance stability was examined during long-term operation of solid oxide fuel cells (SOFCs) with humidified methane (S/C = 0.1) at 1000 degrees C: Two kinds of anode materials were evaluated; i.e., Ni-yttria-stabilized zirconia (Ni-YSZ) and Ni-samaria-doped ceria (Ni-SDC). The thickness of anode was controlled to be ca. 10, 20, and 30 mu m in this study. Deterioration in performance was observed during a constant current operation with 10 mu m-thick Ni-YSZ anode. On the other hand, the cell performance with 20 and 30 mu m-thick Ni-YSZ was stable for 20 h. These differences in performance stability were significantly related to the anode thickness. In addition, the same experiments were conducted for the cells employing Ni SDC anodes. The cell performance was improved obviously during discharge for 20 h regardless of anode thickness. The improvement upon discharge is ascribable to an increase in catalytic activity of Ni SDC and electronic conductivity of SDC, which is induced by the partial reduction of Ce4+ to Ce3+ under reducing atmospheres. Microstructural observation and Raman spectroscopy were also conducted for the anodes. The electrochemical behavior observed in each anode had a strong correlation with crystallinity of carbon deposited and deposition area. (C) 2013 The Electrochemical Society. All rights reserved.