The interaction of O-18(2) With physical mixtures of MoO3 and Sb2O4 and of MoO3 and Sb2O3 has been investigated by in situ Raman spectroscopy. Signal intensity ratios of the two O-18-related high-frequency Raman bands of MoO3 at 788 and 945 cm(-1) relative to the corresponding O-16-related bands at 820 and 995 cm(-1) lead to the assignment of these bands to the two different terminal Mo=O groups along the a and b axes. At elevated temperatures, the scattering cross section of MoO3 is strongly diminished relative to antimony oxides. MoO3 in the mixture with Sb2O4 does not show enhanced O-18 exchange as compared to pure MoO3, suggesting that efficient spillover of oxygen does not occur in this mixture, probably because of the very slow kinetics of that process. Some spillover during the reexchange with O-16(2) may be indicated by a slightly increased signal intensity of the O-18-related bands. As in the pure compounds, the reexchange with O-16 atoms occurs very slowly and is not influenced by the presence of H2O. Significant reduction of MoO3 in contact with Sb2O3, after evacuation at 648 K as compared to pure MoO3 possibly indicates an intimate cross talk between the two phases, by which MoO3 is reduced via a "reverse spillover". Reoxidation of the evacuated sample shows an incorporation of heavy oxygen into the MoO3 lattice. The comparable signal intensities of the O-18-related band and of the line attributed to a suboxide reveal that only oxygen defects are refilled during the O-18 treatment in MoO3. A further oxygen exchange does not seem to take place. In contrast to the reexchange in dry O-16(2), the presence of H2O in the gas phase leads to a distinct decrease of the O-18-related signals. A comparison of the experiments carried out after evaucation at 393 and at 648 K, only shows signals due to O-18 incorporation into MoO3 if MoO3 in the sample had a higher degree of reduction, thus revealing the essential role of lattice defects in the oxygen exchange. The results suggest that in the Sb2O4/MoO3 mixture, surface mobile (spillover) species should be involved to explain the catalytic synergy between the two phases.