The invention comprises novel undoped and doped nanometer-scale CeO₂ particles as well as a novel semi-batch reactor method for directly synthesizing the novel particles at room temperature. The powders exhibited a surface area of approximately 170 m²/g with a particle size of about 3-5 nm, and are formed of single crystal particles that are of uniform size and shape. The particles' surface area could be decreased down to 5 m²/g, which corresponds to a particle size of 100 nm, by thermal annealing at temperatures up to 1000 C. Control over the particle size, size distribution and state of agglomeration could be achieved through variation of the mixing conditions such as the feeding method, stirrer rate, amount of O₂ gas that is bubbled through the reactor, the temperature the reaction is carried out at, as well as heating the final product at temperatures ranging from 150 to 1000 C.