H 2 O/CO 2 co-electrolysis through solid oxide electrolysis cell (SOEC) combined with renewable energy system and Fischer-Tropsch system to produce fuels is a promising way to convert electrical energy into chemical energy. In this work, fuel composition, conversions of H 2 O and CO 2 , selectivity to H 2 , CO and CH 4 , and yields of H 2 , CO and CH 4 for H 2 O/CO 2 co-electrolysis are investigated based on the thermodynamic equilibrium under the boundary condition without carbon deposition. High selectivity to CH 4 can be reached at suitable M H2O /M CO2 ratio and low temperature, while high selectivity to CO and H 2 can be reached at high temperature according to the thermodynamic equilibrium results. When the operating temperature of SOEC is low, the conversions of H 2 O and CO 2 cannot reach high due to carbon deposition, which can be improved by increasing M H2O /M CO2 ratio. When the operating temperature of SOEC is high, the conversions of H 2 O and CO 2 can reach high without carbon deposition, which can be higher than 90% at 1073 K according to the thermodynamic equilibrium results. The syngas with different H 2 /CO ratios for Fischer-Tropsch synthesis of different fuels can be produced by selecting appropriate co-electrolysis condition. • Fuel composition, conversion, selectivity and yield for SOEC are investigated. • High H 2 O/CO 2 ratio should be used to avoid carbon deposition at low temperature. • High CH 4 selectivity can be reached at suitable H 2 O/CO 2 ratio and low temperature. • Different syngas can be produced by selecting suitable operating condition of SOEC. • High CO selectivity can be reached by co-electrolysis at high temperature. [ABSTRACT FROM AUTHOR]