Tertiary carboxylate solutions of cerium(III) were hydrolyzed and precipitated micrometer, crystalline ceria (CeO2) particles, when contacted with both water and atmospheric oxygen at 130-170 degrees C (0.27-0.78 MPa) for 30-180 min. The rate of ceria precipitation in a batch autoclave was markedly dependent on processing parameters such as temperature, liquid-phase stirring speed, and initial organic-phase concentrations of cerium(III) carboxylate and free carboxylic acid. The particle size distributions of the ceria particles were considerably influenced by these processing parameters. In addition, the nature of aliphatic and aromatic hydrocarbon diluents had an appreciable effect on the size distribution of ceria.