A sample Of MoOx/SiO2, in which all of the Mo cations are present as isolated mono-oxo molybdate moieties, was prepared and investigated to understand the redox chemistry of such molybdate species and their ability to exchange oxygen with O-2 and H2O. Raman spectroscopy was used to monitor the exchange of O-18 for O-16 in the Mo=O bond of isolated molybdate species, whereas mass spectrometry was used to follow the isotopic composition of the gaseous species, i.e., O-2 and H2O. Reduction in H-2 at 920 K results in the loss of one O atom per Mo atom, and consistent with this, all of the Mo-VI cations are reduced to Mo-IV cations. Raman spectroscopy shows that virtually all Mo=O bonds of the original molybdate species are lost upon reduction. While reoxidation of Mo-IV cations by O-2 is quantitative, studies using O-18(2) reveal that only a small part of the newly formed Mo=O bonds are O-18 labeled, and that the balance are O-16 labeled, indicating that O-atom exchange between the support, SiO2, and the supported MoOx species occurs during reoxidation. Rapid exchange of O atoms was observed upon exposure of both bare SiO2 and MoOx/SiO2 to (H2O)-O-18 at 920 K, and the presence of MoOx species was found to enhance the rate of exchange. By contrast, very slow exchange of O atoms was observed when the oxidized catalyst was exposed to O-18(2) at 920 K. In situ observations of the catalyst during exposure to a mixture of H-2 and O-18(2) at 920 K showed that all of the Mo cations remained in the VI oxidation state and that O atom exchange occurred at a rate comparable to that observed upon exposure to (H2O)-O-18. The results of this investigation suggest that reoxidation of Mo-IV cations following H-2 reduction involves the formation of a Mo-peroxide species and subsequent O atom migration from such a species to the SiO2 support. It is proposed that the steady-state oxidation of H2 also involves the formation of Mo-peroxide species by interaction of O-2 with a small number of Mo-IV centers. The Mo-peroxide species are then rapidly reduced by H-2 to form H2O and a Mo=O bond. The rapid exchange of O atoms between the gas phase and the catalyst observed during steady-state oxidation of H-2 is attributed to interactions of the product H2O with the catalyst, rather than to O atom migration originating from the Mo-peroxide species formed on the catalyst surface.