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The temporal evolution of the lattice parameter of oxygen deficient nanocrystalline cerium oxide films was monitored by X-ray diffraction. It was found that films with lattice parameter of ≈ 5.47 Å upon deposition undergo large spontaneous expansion, in which the size of the unit cell increases by ≈ 0.7% during the course of days. The films with as-deposited lattice parameter larger or smaller than ≈ 5.47 Å do not show significant changes. This behavior is consistent with the previously suggested hypothesis of an order-disorder transition of oxygen vacancies and can be viewed as its direct experimental confirmation.