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Cerium oxide, 'ceria', is a promising catalytic material with applications in catalytic converters and solid-oxide fuel cells. UV irradiation of aqueous nanoceria results in the photoreduction of Ce(IV) to Ce(III) within the lattice which is expected to be accompanied by the formation of oxygen vacancies to balance the charge of the nanoparticle. However, the adsorption of water at oxygen vacancy sites likely prevents the oxygen vacancy concentration of aqueous nanoceria from being increased through photoreduction. Doping nanoceria with rare earth ions allows its oxygen vacancy concentration to be manipulated; therefore, in the present work, lanthanum-doped polyacrylic acid-capped nanoceria was synthesized in order to investigate the effect of increased oxygen vacancy concentration on the photochemistry of aqueous nanoceria. Lanthanum incorporation was found to decrease the optical bandgap of the nanoceria. Both the lanthanum-doped and pure nanoceria samples were found to accumulate Ce(III) centers upon UV irradiation in pH 10 conditions; however, photoreduction occurred to a much greater extent in the lanthanum-doped sample possibly due to the presence of bulk oxygen vacancies which prevent recombination of bulk Ce(III) centers with photogenerated holes. Lastly, fluorescence emission spectroscopy was used in conjunction with UV-Vis absorption spectroscopy to monitor Ce(III) formation in lanthanum-doped and pure nanoceria upon UV irradiation. Photogenerated Ce(III) emission at 410 nm was found to continuously increase at similar rates in both nanoceria samples throughout the entirety of UV irradiation despite the increased photoreduction of the lanthanum-doped nanoceria sample. This suggests that photogenerated Ce(III) emission at 410 nm may exclusively be attributed to surface Ce(III) centers instead of additional photogenerated bulk Ce(III) centers that may be present in the lanthanum-doped nanoceria sample.