The ammoxidation of propane to acrylonitrile was investigated over a series of V/Sb mixed oxide catalysts, Characterizations of catalysts were carried out by X-ray diffraction, temperature-programmed desorption of ammonia and infrared spectroscopy. The rutile-type of VSbO4 is found to be highly selective for the formation of acrylonitrile due to its enhancing effect of reoxidation. Correlation of the XRD patterns and NH3-TPD profiles indicates that the ammonia desorption peak appearing at about 90 degrees C is caused by the ammonia adsorption on the VSbO4 rutile-type surface. Ammonia inhibits the formation of carbon oxides and acrylonitrile, and therefore the competition between ammonia and oxygen takes place on the same kind of active site. Propane is found to be adsorbed on another site. With regards to the reaction mechanism, it is proposed that the adsorption of propane on the surface occurs first through a formation of propylene-like adsorption intermediate, which appears to react with oxygen to form an oxidized site, The further reaction with ammonia forms a second oxidized site and finally, acrylonitrile is produced from the second oxidized site. A Langmuir-Hinshelwood model fits fairly the experimental data.