Two series of V2O5/TiO2-Al2O3 catalysts were prepared, In the first, a constant loading of vanadium (7.5 wt% V2O5) was deposited on a series of mixed TiO2-Al2O3 supports of various compositions (0-82 wt% Al2O3), whereas in the second series various amounts of vanadium (4.5-10.5 wt% V2O5) were supported on a TiO2-10 wt% Al2O3 support. X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), and IR spectroscopy were used to study the influence of the composition on the structure of the catalysts and on the oxidation state of supported vanadium species. The acidity of the catalysts was studied using IR of chemisorbed NH3. The results showed the presence of surface V4+ species, in addition to V+5 species, on the V2O5/TiO2-Al2O3 catalysts. The ratio V4+/V5+ was found to depend strongly on both the composition of the mixed support and the loading of vanadium. Deposition of vanadium on these mixed carriers results in an increase of the strength of Lewis-acid sites and of the surface concentration of Bronsted-acid sites, whereas the strength of Bronsted- and the surface concentration of Lewis-acid sites are not significantly affected. For constant vanadium loading, the composition of the mixed support strongly influences the surface concentration of both kinds of acid sites, especially in the range 0-10 wt% Al2O3.