Nanocrystalline CeO2 powders have been successfully synthesized via a carbonate precipitation method, using ammonium carbonate (AC) as the precipitant and cerium nitrate hexahydrate as the cerium source. The AC/Ce3+ molar ratio (R) affects significantly precursor properties, and spherical nanoparticles can be produced only in a narrow range of 2 < R less than or equal to 3. The precursor, having an approximate composition of Ce(OH)CO3.2.5H(2)O, decomposes to CeO2 at temperatures greater than or equal to300degreesC. The CeO2 powder calcined at 700degreesC exhibits high reactivity and can be densified to >99% of theoretical at 1000degreesC.