The influence of the V/AI ratio composition on the physico-chemical and catalytic properties of the vanadium aluminium oxynitride system was investigated. The samples were prepared by co-precipitation of vanadium and aluminium solutions containing different metal compositions (0.1-0.9 V/Al) at pH 5.5 and characterized by XRD, XPS, Raman and BET surface area. Catalytic activity measurements for the propane ammoxidation reaction were carried out under optimal acrylonitrile selectivity conditions. X-ray diffraction pattern indicated that all the catalysts in both oxide precursor and nitride state show amorphous character. BET surface area was higher for the sample with V/Al ratio of 0.25, before and after nitridation treatment. This sample showed optimal catalytic performances, with 50% acrylonitrile selectivity and 60% propane conversion. The optimal nitridation degree, which induces an optimal reduction degree of vanadium, would explain the maximal catalytic activity observed for the sample prepared using the V/Al ratio composition of 0.25. (c) 2005 Elsevier B.V. All rights reserved.