In situ Raman spectroscopy was used to investigate the oxygen-exchange properties of MoO3 and the role of bulk defects during redox processes. A change of the absorption coefficients between MoO3 and molybdenum suboxides as evidenced by a change in color from pale yellow to gray blue may contribute to a general decrease in Raman intensities. An alteration of the Raman scattering tenser of MoO3-x, is indicated by additional Raman bands and by changes in signal intensity ratios compared to stoichiometric MoO3. This different scattering tenser may also contribute to the lower Raman efficiency of Mo suboxides. Furthermore, besides temperature-induced broadening and signal shifts, Raman spectra of MoO3 seem to be sensitive to the crystallite size due to sintering and crystal growth. Calcination in O-18(2) after evacuation at 393 K (sufficient to remove physically adsorbed H2O and CO2 from the surface) does not result in any detectable spectral changes due to O-18 exchange. On the other hand, after evacuation at 648 K leading to a higher degree of reduction, calcination in O-18(2) gives rise to bands due to an O-18 incorporation into MoO3. Therefore, the intensity ratio of the O-18-related bands relative to those of the O-18-free stoichiometric oxide may correlate with the initial bulk defect concentration in oxygen-deficient MoO3. The long time required for the O-16 reexchange of MoO3 when fully reoxidized in O-18(2), constitutes a proof of the essential role played by defects in the anion vacancy conductor MoO3 during the exchange process. H2O present in the gas phase has no detectable influence on the O-16 reexchange in MoO3.