gamma-Al2O3-supported vanadium oxide catalysts (0-6.3 wt% V atoms) were prepared, characterized, and tested for the oxidative dehydrogenation (ODH) of both ethane and n-butane. The vanadium loading strongly influences the catalytic behavior of gamma-Al2O3-supported vanadium oxide catalysts during the ODH of n-butane and ethane. While the catalytic activity in the ODH reactions increases with the vanadium loading, the selectivity to the corresponding olefins shows a maximum for catalysts with 3-4 wt% V atoms. In addition, the selectivity to oxydehydrogenation products during the ODH of butane was lower than the selectivity to ethene during the ODH of ethane. Associated VO4 tetrahedra were observed by V-51-NMR on catalysts with V loading lower than 5 wt%, while VO6 octahedra were predominant on catalysts with higher V loading. In this way, although catalysts with octahedral V5+ species show higher catalytic activity, higher selectivity to oxydehydrogenation reactions was achieved on catalysts in which mainly tetrahedral V5+ species were observed. The number and nature of the acid sites also change with the vanadium loading. Thus, Lewis acid sites were observed mainly on catalysts with low V loading, while both Lewis and Bronsted acid sites were observed on catalysts with high V loading. Although V5+ species were observed mainly on calcined samples, V4+ species are observed on tested catalysts and their concentration depends on the alkane conversions used in the catalytic tests.