The electrical conductivity change with annealing at 1000degreesC in the Y2O3-ZrO2 (YSZ) system was examined. Among the sintered samples doped with 8.0 mol% Y2O3 (8.0YSZ), 8.5 mol% Y2O3 (8.5YSZ), 9.0 mol% Y2O3 (9.0YSZ), 9.5 mol% Y2O3 (9.5YSZ) and 10.0 mol% Y2O3 (10.0YSZ), 9.5- and 10.0YSZ showed no conductivity decrease even for the annealing period of 1000 h, while 8.0- and 8.5YSZ showed significant decrease with time although the initial conductivities were higher than those of 9.5- and 10.0YSZ. The 9.0YSZ showed only slight conductivity decrease. The measurement of Raman spectra demonstrated that the deterioration in conductivity related to the gradual formation of fine tetragonal phase in the cubic phase. Consequently, 9.5YSZ was seemed to be optimum as the electrolyte material of solid oxide fuel cells from the point of stability in high conductivity. (C) 2003 Elsevier B.V. All rights reserved.