Monophasic, rutile-type Mo/V/Sb mixed oxides were synthesized with the co-precipitation procedure; Mo6+ replaced for V4+ in the lattice of V antimonate, and the (Mo + V)/Sb ratio remained approximately constant. The addition of Mo increased the catalytic activity in propane ammoxidation, but the catalysts became less selective to acrylonitrile, due to the increased contribution of combustion, and more active in ammonia oxidation to molecular nitrogen. This was attributed to an enhanced Lewis acidity, which increased the interaction of the catalyst with ammonia and favored its unselective transformation. (C) 2004 Elsevier B.V. All rights reserved.