The effect of Te addition over Mo-V-O catalysts supported on alumina is discussed for the ammoxidation of propane to acrylonitrile. Catalyst composition and atmosphere of activation are evaluated. Catalysts are characterized before and after catalytic reaction by XPS, XRD and in situ Raman spectroscopies. The absence of Te in catalysts formulation and the presence of a high amount of vanadium induce the presence of V5+ species and the formation Of V2O5 oxide; associated with a decrease in acrylonitrile selectivity. The presence of Mo-based polyacids structures decreases the selectivity to acrylonitrile. V5+ sites are responsible of propane activation and of the subsequent U-H abstraction to form the intermediate propylene. Then, a Mo-V rutile-like structure in which vanadium species are reduced as V4+, is responsible for nitrogen insertion and acrylonitrile formation. The formation of such structure is favoured when Te is added to catalysts and is promoted during propane ammoxidation. (c) 2008 Elsevier B.V. All rights reserved.