Samaria-doped ceria solid oxide fuel cells (SOFC) single cells have demonstrated power densities >250 mW/cm(2) at 700 degrees C and stable performance for 15,000 h. Though encouraging, issues of stack performance and efficiency due to mixed conduction remain. Stacks operated on H-2/3% H2O show approximately half the power density expected from single cells. A coupled thermal-electrochemical model which included conduction heat transfer in the solid and convection to the reactant gases was developed. The model was used to predict stack efficiencies using ceria-based SOFCs in power plant applications. Analyses indicate that ceria SOFCs may provide as high as 42% electrical efficiency when operated on humidified hydrogen fuel. These efficiencies are surprisingly high and challenge the notion that mixed conductors are unsuitable for such applications.