Vibrational predissociation spectra of rare-gas-tagged [(CeO2)(VO2)(1-2)](+) and [(Ce2O3)(VO2)](+) clusters are measured in the 400-1200 cm(-1) region. Density functional theory (DFT) is used to determine the geometric and electronic structure of low-energy isomers of the partially reduced dusters. Comparison of experimental and simulated spectra provides evidence for the larger stability of Ce+3/V+5 compared to that of Ce+4/V+4, which confirms that the exceptionally high reducibility of Ce+4 accounts for the promoting role of ceria in supported vanadium oxide catalysts.