The addition of Co to VPO formulations improves the yield of n-butane to maleic anhydride. In this work, different modes of impregnation and two different organic cobalt salts were used. The equilibrated catalysts were characterized using XRD, P-31 SEM NMR, FT-IR and acetonitrile adsorption to evaluate Lewis acidity. The best catalyst was obtained using Co acetyl acetonate for impregnation of the VOHPO4.0.5H(2)O precursor. This catalyst after equilibration had an optimum concentration of very strong Lewis acid sites, very low concentration of isolated V(V) centers, and no V(V) phases.