In this study, ultrafine Ce0.8Y0.2-xO2-delta (for x = 0.00, 0.04, 0.08, 0.12, 0.16, 0.20) powders have been successfully prepared by sol gel auto-combustion method. The samples were characterized by FTIR, TG-DSC, XRD, Raman spectroscopy, BET, TEM, SEM, AC impedance, and thermal expansion measurements. The results indicated that Ce0.8Y0.2-xCuxO2-delta powders with highly phase-pure cubic fluorite-type structure were obtained after calcining at 700 degrees C for 2 h; the average crystallite sizes were between 11.3 and 17.9 nm. The as-synthesized powders exhibited high sintering activity, Ce0.8Y0.2-xCuxO2-delta series electrolytes which have higher relatively dense over 95% can obtain after sintering at 1300 degrees C for 4 h Ce0.8Y0.12Cu0.08O1.86 electrolyte sintered at 1300 degrees C for 4 h exhibits the highest oxide ionic conductivity (sigma(800) (degrees C) = 0.029 S cm(-1)), lowest electrical activation energy (E-a = 0.75 eV) and moderate thermal expansion coefficient (TEC = 13.5 x 10(-6) K-1, between 25 and 800 degrees C). Therefore, it was concluded that co-doping with appropriate ratio of Y/Cu can further improve the properties of ceria electrolytes. (C) 2014 Elsevier B.V. All rights reserved.