This paper uses a large elementary reaction mechanism to study the homogeneous chemistry of methane and natural-gas mixed with air and steam. Temperatures and residence times are chosen to represent SOFC operating conditions, including within feed lines that may be at elevated temperature but without any electrochemical or catalytic interactions. Mole fractions of six major species (CH4, O-2, H2O, H-2, CO, and CO2) are presented as contour maps as functions of temperature, residence time, and initial fuel mixture compositions. In addition, deposit propensity is predicted by the sum of mole fractions of all species containing five or more carbon atoms, designated as C5+. Comparison with chemical equilibrium predictions shows that the homogeneous reactions are far from equilibrium. These results indicate that the composition of the fuel mixture entering the active SOFC region might be significantly different from that originally entering the fuel cell. (c) 2005 Elsevier B.V. All rights reserved.