The structures of different titania, vanadia, and vanadia-titania clusters located on the surface of mesoporous silica SBA-15 are optimized using density functional theory (DFT). The apparent absorption spectra arising from these clusters are calculated with the aid of the ORCA package. The silica support is shown to contribute to the absorption spectra at wavelengths much shorter than those observed for vanadia and titania clusters located on the SBA-15 surface. The comparison of calculated and experimental apparent absorption spectra of supported vanadia and titania catalysts reveals that titania species generally show a higher nuclearity compared to vanadia species at similar low loadings. SBA-15 based catalysts loaded with both vanadia and titania are supposed to contain two types of species: species in which the V ions are anchored to the titania ones and those in which V and Ti ions alternate and are mainly coupled to the support through M-O-Si (M = V, Ti) bridges. The latter provide the major contribution to the apparent absorption spectra at not very high Ti loadings.