Anode-supported solid oxide fuel cells (SOFCs) with Ni+yttria-stabilized zirconia (YSZ) anode, YSZ-samaria-doped ceria (SDC) bilayer electrolyte, and Sr-doped LaCoO3 (LSC) + SDC cathode were fabricated. Fuel used consisted of H-2 diluted with He, N-2, H2O, or CO2, mixtures of H-2 and CO, and mixtures of CO and CO2. Cell performance was measured at 800degreesC with the above-mentioned fuel gas mixtures and air as oxidant. For a given concentration of the diluent, cell performance was higher with He as the diluent than with N-2 as the diluent. Mass transport through porous Ni-YSZ anode for H-2-H2O, CO-CO2 binary systems, and H-2-H2O-diluent gas ternary systems was analyzed using multicomponent gas diffusion theory. At high concentrations of diluent, the maximum achievable current density was limited by the anodic concentration polarization. From this measured limiting current density, the corresponding effective gas diffusivity was estimated. Highest effective diffusivity was estimated for fuel gas mixtures containing H-2-H2O-He mixtures (similar to0.55 cm(2)/s), and the lowest for CO-CO2 mixtures (similar to0.07 cm(2)/s). The lowest performance was observed with CO-CO2 mixture as a fuel, which in part was attributed to the lowest effective diffusivity of the fuels tested and higher activation polarization. (C) 2003 The Electrochemical Society.