Oxygen vacancy levels are monitored during the oxidation of CO by CeO2-delta nanorods and Au-CeO2-delta nanorods, nanocubes, and nanopolyhedra by using Raman scattering. The first-order CeO2 F-2g peak near 460 cm(-1) decreases when this reaction is fast (fast reduction and relatively slow reoxidation of the surface), because of the lattice expansion that occurs when Ce3+ replaces Ce4+ during oxygen vacancy creation. This shift correlates with reactivity for CO oxidation. Increases in the oxygen deficit delta as large as similar to 0.04 are measured relative to conditions when the ceria is not reduced.