Vanadium species formed on the alumina support after calcination between 773 and 973 K at vanadium loadings between 4.45 and 13.2 wt.% were characterized with different methods (V-51 NMR, ESR, H-2-TPR and in situ UV-vis-DRS) and tested in the oxidative dehydrogenation of propane. By calcination at 873 K, beside dispersed isolated and polymerized V5+ surface species mainly in tetrahedral coordination, a crystalline AIVO(4) phase was formed. The portion of vanadium present as AIVO(4) increases with increasing vanadium loadings (e.g. about 60 at 13.2 wt.%). The domain size of the polymerized V5+ species is only weakly influenced by the vanadium loadings at calcination up to 873 K. It is suggested that both isolated and polymerized tetrahedrally coordinated V5+ species are active in propane dehydrogenation. The AIVO(4) phase and bulk-like V2O5 have only a low catalytic activity. The highest propene selectivity was obtained for the sample with the highest dispersion of the isolated and polymerized tetrahedrally coordinated V5+ species. (C) 2004 Elsevier B.V. All rights reserved.