A Gd and Mg co-doped ceria electrolyte Ce0.85Gd0.1Mg0.05O1.9 (CGM) was prepared by sol-gel method. Its conductivity behavior was studied and compared with those of singly doped ceria electrolytes Ce0.9Mg0.1O1.9 (CM10) and Ce0.9Gd0.1O1.95 (CGO). By using CGM as the electrolyte, a fuel cell was fabricated with a Ni/CGM anode and a La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)/CGM composite cathode. Its performance was compared with that of CGO based fuel cells. Both the CM10 and CGM electrolytes were ceria based solid solutions. The dopant Mg2+ might be situated in the interstitials of the crystal lattice of CeO2. The co-doped CGM showed a higher conductivity in air at 773-973 K in comparison to that of CGO and CMO. The CGM based fuel cell also showed higher open circuit voltage (OCV) and higher maximum power density (MPD) than the CGO based cell at 773-973 K. The MPD of the fuel cells were improved further by using composite cathode of LSCF/CGO (or CGM). With Ni/CGM anode and LSCF/CGM cathode, the MPD of the CGM based fuel cell at 873 K reached 202.5 mW cm(-2). (C) 2004 Elsevier B.V. All rights reserved.