The effect of Mg as dopant on alumina-supported mixed Sb-V oxide catalysts during the ammoxidation of propane reaction is evaluated. The Mg-doped catalyst increases acrylonitrile selectivity compared to the un-doped Sb-V-O/Al2O3 system. The presence of magnesium facilitates the formation of a cationic vacancies enriched rutile VSbO4 phase, involved in C-N triple bond formation. In addition, Mg increases isolation of V5+ structural site isolation, which may affect selectivity.