The direct current four-probe method has been employed to investigate the conduction of oxide ion and proton in a doped ceria-carbonate composite electrolyte for fuel cells. The measurements are conducted in oxygen and in hydrogen atmospheres in the temperature range of 425-650 degrees C. The conductivities of both of O2- and H+ increase with the increase of carbonate content above the melting point of the carbonate. The ionic conductivities of the composite electrolytes have also been simulated using the effective medium percolation theory. The deviations between experimental results and simulated values of O2- conductivity are caused by the associating effect of ceramic and carbonate phases, which leads to a higher O2- migration energy through the phase interface. According to the comparison of experimental data and simulated values, the conduction mechanisms of O2- and H+ have been proposed. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.