The laser Raman spectra of MoO3 and NiO-MoO3 supported on gallia and gallium-aluminum mixed oxides are presented and correlated to the type of support and the hydrodesulfurization (HDS) activity of the catalysts. The results show that the support affects the species (orientation of catalyst) that are formed on its surface. For example, MoO3 forms mainly HDS inactive tetrahedral species on gallia; on alumina surface, it forms the HDS active polymeric octahedral species. Also, bulk oxide is formed more readily on gallia than on alumina. On addition of NiO, as a promoter, to the MoO3 catalyst supported on gallia, a band that can be assigned to HDS inactive species was also formed. The intensity of this band is low in the spectra of NiO-MoO3 catalysts supported on alumina.