Aging effect of dense 8 mol % yttria-stabilized zirconia (8YSZ) ceramics with different microstructures has been studied. Spark-plasma sintered (SPS) ceramics consisting of submicrometer grains showed relatively slow conductivity degradation with aging time compared with conventionally sintered ceramics. Conductivity degradation was attributed to the increase in the bulk resistivity, which would be due to subtle structural changes in the 8YSZ unit cells. SPS ceramics consisted of relatively small crystallites, which might be responsible for the relatively slow conductivity degradation. (C) 2004 The Electrochemical Society.