A one-dimensional elementary reaction kinetic model for solid oxide fuel-assisted steam electrolysis cell (SOFEC) is developed coupling heterogeneous elementary reactions, electrochemical reaction kinetics, electrode microstructure and transport processes of charge and mass. This model is calibrated and validated by experimental data from a button cell with anode gases of H-2, CO and CH4 at 800 degrees C. After comparisons with solid oxide electrolysis cell (SOEC), the energy demands, performance and efficiency of CO-assisted SOFEC and CH4-assisted SOFEC are investigated numerically. One important finding is that over 80% of electricity can be saved by SOFEC at a current density of 3000 A m(-2). SOFEC assisted by CO or CH4 for steam electrolysis has better performance than SOEC, especially by CH4. The efficiencies of 12% CO-SOFEC and 12% CH4-SOFEC are at least, respectively, 7% and 30% higher than that of SOEC at 800 degrees C with the current density of below 2500 A m(-2). Finally, the effects of type of assisting-fuel, fuel composition and applied voltage are studied. It is found that CO-SOFEC shows higher anode polarization and thus lower performance than CH4-SOFEC with the same molar fraction of fuel. It is also found that the performance of SOFEC increases with increasing proportion of assisted fuel in anode at high current density. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.