The selective oxidation of propene to acrolein over USb3O10 was studied in a continuous flow reactor. Our experimental results show that the preparation of a well-defined catalyst of uniform composition depends on the temperature and time of calcination. The kinetic data indicate that acrolein is formed via a redox mechanism in which the surface of the catalyst is partially reduced. Mossbauer spectroscopy reveals the presence of Sb5+ and Sb3+ in the used catalyst. I.r. spectroscopy shows a structural modification.On the basis of these results we conclude that each steady-state condition is characterized by a vacancy concentration leading to the observed modifications of the catalyst.