Ce0.8Sm0 15R0.05O2-delta (R = Sm, Mg, Ca, Sr, and Ba) specimens were successfully prepared using a solid-state reaction, and they were used in an intermediate temperate solid oxide fuel cell (IT-SOFC) electrolyte. This study focused on the effects of co-doping and an aging treatment for the conductivities and microstructures of CeO2-based ceramics and also analyzed the variation of the conductivity in the reducing atmosphere. The study showed that the conductivities of the CeO2-based materials have a higher conductivity at 500-800 degrees C by the co-doping aliovalent cations Sm and R. The conductivity increased with the increasing oxygen vacancies that were induced from charge. compensation. The XRD and EDS analyses showed that the MgO and BaCeO3 phases appeared in the Ce0.8Sm0 15Mg0.05O2-delta and Ce0.8Sm0 15Ba0.05O2-delta specimens, respectively. The conductivity of the Ce0.8Sm0 15Ca0.05O2-delta specimens was higher, approximately 0.0837 S/cm at 800 C in the air. The thermal expansion coefficient (TEC) in all samples was ca. 11-15 x 10(-6)/degrees C at 200-800 degrees C. After an aging treatment at 700 degrees C for a holding time of 1000 h, the conductivities of all samples showed almost no change. However, the conductivity in Ce0.8Sm0 15Ca0.05O2-delta decreased from 0.0837 to 0.0581 S/cm, and the grain size increased. The conductivities of the CeO2-based specimens were also measured under a 5%H-2-95%N-2 atmosphere, and the conductivity greatly increased in the reducing atmosphere because the Ce4+ ions reduced to Ce3+ ions. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.