Co-doped ceria of Ce1-xGdx-yYyO2-0.5x, wherein x = 0.15 and 0.2, 0 <= y <= x, were prepared by glycine-nitrate method. Their structures and ionic conductivities were characterized by X-ray diffraction (XRD) and AC impedance spectroscopy (IS). All the electrolytes were found to be ceria-based solid solutions of fluorite type structures. Co-doping was found to effectively enhance the conductivity. In comparison to the singly doped ceria, the co-doped ceria showed much higher ionic conductivities at 673-973 K. At 773 K, the ionic conductivity of Ce0.8Gd0.05Y0.15O1.9 is 0.013 S cm(-1) which is three times as high as that of Ce0.8Gd0.2O1.9. These Gd3+ and Y3+ co-doped ceria are ideal electrolyte materials of intermediate temperature solid oxide fuel cells (SOFCs). (C) 2007 Elsevier Ltd. All rights reserved.