The electrical conductivity and phase stability of ZrO2 co-doped with Sc2O3 and CeO2 were investigated in order to search for a better solid electrolyte material, which can be applicable for intermediate- or low-temperature solid oxide fuel cells (SOFCs). Present study showed that ZrO2 doped with Sc2O3 and CeO2 did not show any phase transition during heat treatment up to 1250-1550 degreesC and was exceedingly stable as a cubic phase in all temperature ranges. The ZrO2 co-doped with Sc2O3 and CeO2 showed much higher electrical conductivity than YSZ in the whole temperature range of 300-1100 degreesC and better long-term stability than any other Sc-ZrO2-based electrolyte. From the long-term performance check of the 5 x 5 cm(2) unit cell operation, we could verify that ZrO2 co-doped with Sc2O3 and CeO2 can be a strong candidate electrolyte material for intermediate- or low-temperature SOFC. (C) 2004 Elsevier B.V. All rights reserved.