The nature and redox properties of vanadium species on metal oxide supported vanadium catalysts have been studied. The calcined metal oxides-Al2O3, MgO, Mg-Al-hydrotalcite (Al/Mg = 0.33), and Mg-Al-Spinel (Al/Mg = 3.0)-have been employed as supports. TPR and IR spectra of adsorbed CO show a different reducibility of the vanadium species related to the acid-base character of the metal oxide support. This is also confirmed by the EPR results of both calcined and reduced catalysts. A reduction of V5+ to the V3+ species seems to be more favored on acid metal oxides. On basic metal oxide supports, the reducibility seems to be hindered, leading preferentially to the formation of V4+, although V3+ ions are also formed at higher reduction temperatures. A shift in the carbonyl frequency of the V4+-CO and V3+-CO complexes to lower frequencies by increasing the basicity of the support could be related to an electron donor effect of the support on the electron density of the vanadium species. In addition, a higher reoxidation ability of the V4+ and V3+ species is observed on the more acidic supports than on the basic ones.