The ultraviolet-visible-near IR spectra of a series of catalysts have been recorded and discussed. In particular bulk and alumina-, titania-and silica-supported V, W, and Mo oxides have been considered. Additionally mixed V-W and V-Mo-supported oxides have been investigated. The data show that, in agreement with vibrational data and other literature data, the oxides supported on silica are similar to the corresponding bulk oxides. In contrast, the spectra of the oxides supported on alumina and titania correspond to surface oxide species where the metal stays in a lower overall coordination with respect to the bulk oxides. Finally, the spectra show that only in the case of titania-supported oxides an electronic interaction between the supported metal oxide centers through the support conduction band is possible. This allows to justify on electronic bases the activating effect of titania for vanadia, vanadia-tungsta and vanadia-molybdena catalysts used in the hydrocarbon selective oxidation catalysis and for the selective catalytic reduction of nitrogen oxide by ammonia.