Laser Raman spectroscopy is used to study the structure of molybdenum and cobalt species present in Co---Mo/¿-Al2O3 catalyst systems. From comparison with Raman spectra of Mo and Co in known structures it is derived that these catalyst systems contain Mo and Co in different modifications depending on the degree of surface coverage. In the absence of Co, four different Mo species are found. At low coverages isolated molybdate tetrahedra are observed. Increasing the surface coverage results in formation of a polymolybdate phase in which Mo is octahedrally surrounded. At higher coverages "bulk" aluminum molybdate is formed. At very high coverages formation of "free" MoO3 occurs. In Co/¿-Al2O3 samples the color indicates the presence of Co3O4- and CoAl2O4-like species. When Co is introduced in Mo/¿-Al2O3 (Co/Mo atomic ratio, 0.64) various effects occur. "Free" MoO3, as well as Al2(MoO4)3, is converted into "CoMoO4." Cobalt addition results in a decrease of the isolated Mo tetrahedra concentration in favor of the polymeric molybdate form, which apparently is not qualitatively affected by the presence of Co. In Co---Mo/¿-Al2O3 most of the Co is present in a structure comparable to CoAl2O4. The influences of the nature of the support, heat treatment, reduction in hydrogen and the effect of sulfiding are discussed briefly.