The effect of annealing on the electrical conductivity of the Y2O3-ZrO2 system (8.0-10.0 mol% Y2O3) was investigated by the direct current four-probe technique at several temperatures for 1000 h. The difference of the crystal structure before and after annealing was investigated by X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). The decrease of the conductivity for the specimens with composition of 8.0-9.0 mol% Y2O3 was caused by the formation of a fine tetragonal phase. On the other hand, the sample of 9.5 mol% Y2O3 after annealing for 1000 h supported a single phase with cubic structure. Therefore, the optimum composition as electrolyte for solid oxide fuel cell in the Y2O3-ZrO2 system was considered to be 9.5 mol% Y2O3 from the viewpoint of long-term stability with the relatively high conductivity. (C) 2003 Elsevier B.V. All rights reserved.