Nanograde cerium oxide particles were successfully formed by a normal-pressure hydrothermal process. Seeds were first prepared using urea precipitated from a urea solution forming a yellow transparent suspension. The primary particle size of the seeds was established by a transmission electron microscopic observation. After seed formation, nanoparticle-sized cerium oxide was produced by two methods. The first method involved urea as the precipitant and the second involved ammonium cerium nitrate as the precipitant. When urea was the precipitant, the mean particle size of cerium oxide increased as the seed concentration decreased. The final product was an aggregation of particles with a secondary particle size of similar to20-30 nm. In contrast, when ammonium cerium nitrate was the precipitant, the mean particle size of cerium oxide increased as the formation temperature increased or as the seed concentration increased. The secondary particles of the second process were aggregates. The differences between these two processes, such as the formation temperature, the seed concentration, and the morphology of the final products, are discussed in this paper. (C) 2004 Elsevier B.V. All rights reserved.