Solid electrolytes are the most important and indispensable part of a solid oxide fuel cell (SOFC) where hydrogen is used as one of the fuels to obtain electricity. Ce0.9Gd0.3O1.95 and Ce0.9Sm0.1O1.95 were chosen to be the base electrolytes. The effects of MgO and Nd2O3 as co-dopants on the electrical conductivity were investigated, respectively. For 4 mol% Mgdoped Ce0.9Gd0.1O1.95 or Ce0.9Sm0.1O1.95, MgO phases were detected by FESEM micrographs, which showed a very low solubility of Mg2+ in ceria lattice. The existence of MgO phases was observed to have negligible effect on the grain conductivity, but improve the grain boundary conductivity measured by ac impedance spectroscopy. However, when Nd2O3 was used as a co-dopant, XRD patterns and FESEM both indicated a pure cubic phase. Ce0.9Gd0.05Nd0.05O1.95 and Ce0.9Sm0.05Nd0.05O1.95 were found to exhibit higher grain conductivity, comparing with single-doped ceria. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.