In recent years, ceria-based composites (CBCs) have been developed as electrolytes for low-temperature solid oxide fuel cells. These materials exhibit extremely high ionic conductivities at . It has also been found that both oxide ion and proton can be conducted in the CBC electrolytes, which makes such co-ionic conducting fuel cell distinct from any other types of fuel cells. In this study, a model involving three charge carriers (oxide ion, proton, and electron) is developed to describe the fuel cell with CBC electrolytes. Various operating characteristics of the fuel cell with CBC electrolytes are investigated, compared to those of the fuel cell with doped ceria electrolytes. The results indicate that the CBC electrolyte behaves as a pure ionic conductor, the cell is more efficient, and a higher output is expected at low temperatures under the same pressure operation than that of the cell with doped ceria electrolytes.