Nanoscale cerium oxide is of increasing interest in catalysis, biomedicine, renewable energy, and many other fields. Its versatility derives from the ability to form nonstoichiometric oxides that include both Ce3+ and Ce4+ ions. This work describes oxidation and reduction reactivity of colloidal cerium oxide nanocrystals, termed nanoceria, under very mild solution conditions. For instance, the as-prepared nanoceria oxidizes hydroquinone to benzoquinone, with reduction of some of the Ce4+ ions. Highly reduced nanoceria, prepared by UV irradiation in the presence of ethanol, oxidize hydroquinone back to benzoquinone. This and related reactivity allow tuning of the average cerium oxidation state in the nanocrystals without changes in size or other properties. The amounts of Ce3+ and Ce4+ in the nanoceria were determined both by X-ray absorption spectroscopy and from the stoichiometry of the reactions, measured using 1H NMR spectroscopy. The results demonstrate, for the first time, that the optical absorbance of nanoceria is linearly related to the percent Ce3+ in the sample. The decrease in absorption (blue-shift of the band edge) is due to increasing amounts of Ce3+, not to a quantum confinement effect. These findings demonstrate the facile solution reactivity of nanoceria and establish UV-visible spectroscopy as a powerful new tool for in situ determination of Ce oxidation states in ceria nanomaterials.