In the present communication we report the UV-Vis spectra of VOx/SBA-15, TiOx/ SBA-15 and mixed (VOx)n–(TiOx)n/SBA-15 catalysts with the aim to reveal the underlying molecular structure of the surface oxide species. The motivation arises from the fact that the interpretation of UV-Vis spectra of this type of catalysts in the literature is essentially based on qualitative examination and comparison with molecular or crystalline reference compounds. 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 [1]. The reliability of the conclusions performed on the basis of DFT calculations is supported by the nicely reproduced spectrum of TiO2. The comparison of the observed spectra and those belonging to individual species gave a possibility to make a conclusion about the type of species present on the surface of the fresh catalyst after dehydration at elevated temperature. Generally, it becomes clear that not a single, well-defined species is present, but the spectra seem to reflect superimposed signals of a number of species, which is in agreement with the amorphous nature of the SBA-15 support. 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. Based on the present analysis it was found that on the surface of TiOx/SBA-15 catalysts binuclear, trinuclear, tetranuclear and even octanuclear titania clusters are present with high probability. Interestingly, the nuclearity of vanadia clusters at the surface of SBA-15 seems to be generally lower (mainly bi-, and trinuclear, perhaps also tetranuclear) compared to the nuclearity of the corresponding titania clusters at comparable metal oxide loading. This is, in turn, reflected in a broader appearance of the absorption spectra of VOx/SBA-15 compared to TiOx/SBA-15. At the same time the observed spectra for the catalysts 3Ti/SBA-15 (3wt% Ti loading), 8Ti/SBA-15 (8wt% Ti loading), 1V/SBA-15 (1 wt% V loading) and 4V/SBA-15 (4 wt% V loading) fall in the same range of wavelengths since the titania clusters contain a larger number of promoters.