Nanoceria was produced by an aqueous precipitation technique in the presence of an organic stabilizer. The stable suspensions were diafiltered to remove reaction byproducts. Particles were characterized by transmission electron microscopy with images used to size the particles, and selected-area electron diffraction was used to determine the lattice structure and the lattice constant. For all particles studied, the electron diffraction data were consistent with that of CeO2 and not Ce2O3, as predicted by some researchers for very small particles sizes. At particle diameters of !1 nm, the lattice expansion approached 7%. In agreement with earlier researchers, we interpret this effect as due to the formation of substantial amounts of Ce3+ with corresponding oxygen vacancies, but within the fluorite lattice structure of CeO2. Even at a particle size of 1 nm, there was a measurable oxygen storage capacity, consistent with a still-reducible CeO2 structure, rather than the fully oxidized Ce2O3.