The changes that take place in a vanadium antimonate catalyst during catalytic propane ammoxidation were studied by transient reactivity experiments and infrared spectroscopy. The investigation was focused on both the change in catalytic behaviour of a fresh sample and the change in reactivity with stepwise increases in the concentration of one or more reagents while maintaining the concentration of the other reactants constant. The results obtained show that two main types of modifications are present : (1) A change in the reactivity from the fresh to the activated samples due to the surface formation of a VSbO4 phase by reaction of vanadium and antimony oxides that have not reacted during calcination; this modification leads to formation of carbon oxides and a decrease in the rate of propane depletion as well as a considerable increase in the rate of acrylonitrile formation. (2) A change in the reactivity of the activated sample which is a function of the redox characteristics of the reagent mixture; this modification does not change the rate of propane depletion, but rather only results in differences in the relative selectivity to products. The results also show that (i) in the steady state, the activated catalyst is partially reduced in comparison with the same sample after treatment with oxygen and (ii) the rate of acrylonitrile formation is lower. Infrared results confirm the formation of oxygen vacancies in the catalyst upon interaction with ammonia and propene as well as the presence of these defects in the sample after the catalytic tests.