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The phases of ceria–zirconia nanoparticles observed in air are studied as a function of particle size and composition by X‐ray diffraction, transmission electron microscopy, and Raman spectroscopy. The emergence of two tetragonal phases t″ and t monotonically moves toward higher zirconia concentrations with decreasing particle size. A smaller particle size increases the solubility of zirconia in cubic ceria, while higher zirconia content in ceria stabilizes against coarsening. In particular, the cubic Ce_1−xZr_xO_2−y is persistent and is 8% in phase amount even at 90% zirconia with 33 nm crystal size. Neither the monoclinic phase m nor the tetragonal phase t′ is observed in the present nanoparticles (<40 nm). The effectiveness of these nanoparticles as oxygen source‐and‐sink in catalytic support is largely due to the persistence of the cubic and the t″ phases.