Cerium oxide nanoparticles (CNPs) are of significant interest to the scientific community due to their widespread applications in a variety of fields. It is proposed that size-dependent variations in the extent of Ce3+ and Ce4+ oxidation states of cerium in CNPs determine the performance of CNPs in application environments. To obtain greater molecular and structural understanding of chemical state transformations previously reported for ceria of ≈3 nm nanoparticles (CNPs) in response to changing ambient conditions, micro-XRD and Raman measurements were carried out for various solution conditions. The particles were observed to undergo a reversible transformation from a defective ceria structure to a non-ceria amorphous oxyhydroxide/peroxide phase in response to the addition of 30% hydrogen peroxide. For CNPs made up of ∼8 nm crystallites, a partial transformation was observed, and no transformation was observed for CNPs made up of ∼40 nm crystallites. This observation of differences in size-dependent transition behavior may help explain the benefits of using smaller CNPs in applications requiring regenerative property.