In situ ultraviolet Raman spectroscopy was used to follow the preparation of Mo/H-ZSM5 catalysts by thermal treatment of a physical mixture of MoO3 and H-ZSM5. The Raman spectra recorded from 300 to 1800 cm(-1) during the thermal treatment showed that MoO3 spread over the zeolite support to form dispersed molybdenum oxide species. Curve-fitting analysis of the overall spectral profile for the samples after treatment at 600 degreesC in O-2 showed the presence of three spectral components at 962. 868 and 815 cm(-1) The band at 815 cm(-1) was assigned to the zeolite support. The most intense band at 962 cm(-1) could be attributed to terminal Mo=O groups of isolated tetrahedrally coordinated monomers and the weaker band at 868 cm-l to bridging Mo-O-Mo groups of dimeric species. These last two assignments are not definitive and may be due to other unique structures of oxides of molybdenum on the zeolite framework.