The structure of lanthanide-doped (La, Sm, and Ce) V2O5/Al2O3 catalysts for the Selective Catalytic Reduction (SCR) of NOx with NH3 in the presence of O-2 have been studied by means of XRD, S-BET, XAS, UV-vis, XPS, and TPR techniques. As function of the vanadium loading and lanthanide dopant used, an alteration of the electronic density of support cations and supported vanadium ions is detected. The activation energy of the catalysts in the SCR reaction is directly related with the Lewis acidity of the vanadium species, in such a way that the higher the ionicity of the V-O bond, the higher the specific activity of the catalysts.