Raman spectroscopy has been used to characterize various MoOJy-Al,O, and COO (or NiO)-MoO,/y-AI,O, catalysts containing 12 wt% MOO, and 3 wt% COO (or NiO) prepared by different methods. No significant differences between samples prepared by the wet impregnation and the pore volume method could be observed in molybdena-supported catalysts; in these samples the majority of Mo is present as isolated distorted MOO:tetrahedra due probably to a low coverage of the alumina surface (SBET = 240 mz g-'). Addition of NiO to the molybdena-supported catalysts seems to cause only a slight increase in the distortion and interaction between MOO:species. In contrast, COO addition to the molybdena-supported catalysts changes considerably the spectra features, suggesting that the Mo species are highly distorted, presumably in different surroundings on the alumina surface, due to the strong interaction of Co with Mo species. However, simultaneous impregnation of both MOO, and COO or NiO results in a more regular arrangement of Mo species and an increase in formation of polymeric Mo species, particularly with Co-containing catalyst, where a relative higher degree of polymeric aggregation may occur.