Ionic conductivities of CeO2:Y2O3 bulk ceramic were investigated with different sintering temperatures and correlated with the resulted microstructure. Lower sintering temperatures (T less than or equal to 1400 degreesC) were found to give much higher overall DC conductivities (e.g. sigma (DC) similar to7 X 10(-3) S/cm at 700 degreesC for a 4% Y2O3-doped sample sintered at 1400 degreesC). The samples sintered at lower temperatures showed higher grain boundary conductivities than those sintered at the traditional sintering temperature, 1500 degreesC, The model involving non-resistive grain boundaries can be employed to explain the lower grain boundary resistivities in our samples of low sintering temperatures. These samples were examined by scanning transmission electron microscopy (STEM) with energy dispersive X-ray (EDX) and electron energy loss spectroscopy (EELS). Most of the boundaries (> 90%) were found precipitate-free in the small grain samples, and a higher Y/O ratio was observed at all these boundaries examined. The lower sintering temperatures suppress grain growth giving rise to small grain size (below 1 mum) The finer grain size provides large grain boundary areas for impurities to precipitate and solutes to segregate. Under such condition, there are insufficient impurities to form continuous precipitate layers at all boundaries, such as ion transport blocking layers at boundaries are not fully formed. At the same time, there are insufficient Y'(Ce) ions for all the boundaries in the fine grain samples while the mobility of the Y'(Ce) ions is low at low sintering temperatures to form well developed space charged regions at these boundaries to abate transboundary ionic transport. These three combined effects have abated some of most resistive mechanisms for ionic transport across boundaries.