The electric efficiency and electrochemical properties of anode-supported single cells with humidified methane as a fuel have been investigated using the single cells and anode half-cells. The performance of the single cell which had a relatively low porosity of approximately 37% in the anode substrate with humidified CH4 fuel was found to be lower than that with humidified H-2 fuel. In contrast to this result, the performance of the single cell having a relatively high porosity in the anode substrate with humidified CH4 fuel was found to be almost equal to that with humidified H-2 fuel. It could be concluded that the difference in the performance of the single cell having relatively low porosity in the anode substrate with humidified CH4 fuel and humidified H-2 fuel was caused by the diffusion resistance of CO or CO2 in the substrate. The single cell having relatively high porosity in the anode substrate can be operated at high fuel utilization giving a methane-to-electricity conversion efficiency of as high as 53.6% (HHV). (C) 2004 Elsevier B.V. All rights reserved.