Niobic acid, H8Nb6O19. xH(2)O, was synthesized and studied for its acidic features as a function of its dehydroxylation extent. It was observed to be strongly acidic, using NH3 adsorption calorimetry and isopropanol conversion reaction as probe techniques, and to be weakly acidic on its dehydrated form, Nb2O5. The mixed oxide Al2O3:Nb2O5 in 1:1 molar ratio prepared from aluminum and niobium oxalates was shown to be amorphous up to 1023 K where it crystallized in the form of AlNbO4 and to exhibit higher acidity than Nb2O5 dehydrated, as expected from Tanabe’s model but much less acidic than the niobic acid form. The single Nb2O5 and mixed Al2O3:Nb2O5 oxides were used as supports for grafting MoOx and VOx species, respectively, It was observed that for MoOx/Nb2O5 samples new strong acid sites were created while the redox properties were not as satisfactory as expected. For VOx/Al2O3:Nb2O5 samples, at relatively low coverage (ca. 15%), acidic features were not appreciably modified (only slightly enhanced) while new redox features were generated resulting in a rather satisfactory mild oxidation catalyst. The degree of condensation of VOx species on the mixed oxide surface, including some possible epitaxial grafting, even if not clearly characterized, appeared to play a determining role in oxidative dehydrogenation properties. This constitutes a new example of structure sensitivity in partial oxidation reactions on oxides.