Abstract: The catalytically active components of a series of Fe_2O_3-Sb_2O_4 mixed oxide catalysts with differentSb/Fe ratio,calcined at elevated temperature,as well as with or without support,have been investigated byX-ray diffraction analysis,Mossauer spectroscopy and XPS in order to elucidate their catalytic behavior inammoxidation of propylene.Correlations of acrylonitrile yield with Fe~2+ and Fe~3+ respectively showed that,FeSbO_4 seems to bethe basic active component of catalyst,upon which the formation of new phase containing Fe~3+ is responsiblefor a good selective oxidation.