The effect of the nature and distribution of VOx species over amorphous and well-ordered (MCM-41) SiO2 as well as over gamma-Al2O3 on their performance in the oxidative dehydrogenation of propane with O-2 and N2O was studied using in situ UV-vis, ex situ XRD and H-2-TPR analysis in combination with steady-state catalytic tests. As compared to the alumina support, differently structured SiO2 supports stabilise highly dispersed surface VOx species at higher vanadium loading. These species are more selective over the latter materials than over V/gamma-Al2O3 catalysts. This finding was explained by the difference in acidic properties of silica- and alumina-based supports. C3H6 selectivity over V/gamma-Al2O3 materials is improved by covering the support fully with well-dispersed VOx species. Additionally, C3H6 selectivity over all materials studied can be tuned by using an alternative oxidising agent (N2O). The improving effect of N2O on C3H6 selectivity is related to the lower ability of N2O for catalyst reoxidation resulting in an increase in the degree of catalyst reduction, i.e. spatial separation of active lattice oxygen in surface VOx species. Such separation favours selective oxidation over COx formation. (c) 2005 Elsevier B.V. All rights reserved.