The structure of vanadium oxide species supported on ceria (VO x /CeO2) was investigated under various conditions by in situ multiwavelength Raman spectroscopy, IR spectroscopy, isotopic labeling, and temperature-programmed reduction (TPR). For the first time, the detailed structure of dehydrated VO x species was revealed on the polycrystalline ceria support. VO x species can coexist on ceria surface in the structure of monomer, dimer, trimer, polymer, crystalline V2O5, and CeVO4 as a function of VO x loading. These species interact strongly with both the defect sites and labile surface oxygen of ceria, passivating the redox property of ceria. Under ambient condition, the dispersed VO x species are hydrated into polyvanadate species that can be reversibly dehydrated back to the original structure forms. The ceria support with defect sites facilitates the interaction between water (H2 18O) and V16O x species, leading to very facile isotopic oxygen exchange between the two even at room temperature. During H2 reduction, both the VO x species and the ceria support can be reduced with ceria surface being more reducible. The reducibility of various dispersed VO x species scales with their polymerization degree, that is, polymer > trimer > dimer > monomer. The reoxidation of reduced VO x species is found to proceed via ceria lattice oxygen instead of the gas phase oxygen where ceria acts as an oxygen buffer. The revealed structure evolution of surface VO x species on ceria under hydrated, dehydrated, reduced, and regenerated conditions provides a basis for understanding the vanadia-ceria catalysis.