The structure of Ce3+, which is responsible for the low-temperature oxygen storage capacity of ceria, was determined by high-energy-resolution fluorescence detected X-ray absorption spectroscopy at the Ce L3 and L1 edges. Well-defined ceria nanoparticles (rods, truncated octahedra, and cubes) were synthesized hydrothermally and promoted by platinum nanoparticles. The electronic structure of Ce3+ does not depend significantly on the nature of the exposed crystallographic planes of CeO2 particles; it does, however, differ from the electronic structure of known stable compounds containing Ce3+ ions, such as CeAlO3, Ce(NO3)3·6H2O, and Ce2Zr2O7. Theoretical simulation of Ce L1 and L3 X-ray absorption spectra, quantitative analysis of the oxygen storage capacity, and X-ray diffraction data suggest that Ce3+ ions form both at the surface and in the near-surface layer. Surface and bulk Ce3+ ions are characterized by elongated Ce–O distances in the first coordination shell and almost the same Ce–Ce distances in the second coordination shell with respect to Ce4+ in stoichiometric CeO2. Ce3+ ions on the surface of the nanoparticles surface may have a smaller number of oxygen neighbors (as low as six), while in the near-surface layer they tend to have an 8-fold coordination, thus producing oxygen deficit structures similar to Ce11O20.