The mechanism of the direct ammoxidation of propane is investigated on a Sb/V mixed oxide catalyst (atomic ratio 5/1) in order to understand the sequence of the intermediates and the role of ammonia in the formation of acrylonitrile. The sample was characterized by different techniques (BET, FTIR, XRD and XPS) and tested in different conditions (oxidation and ammoxidation of propane and propene). It is proposed that propane is first converted to propene and hence to "oxidation products" (which could be acrolein or an intermediate absorbed an the catalyst surface, as allyl radical). In the absence of ammonia these "oxidation products" are converted to carbon oxide. But in its presence they are converted into acrylonitrile, which is appreciably more stable and undergoes minor degrees of oxidative degradation.