The effect of niobium dopant on Sb-V oxide catalysts supported on alumina is reported with the purpose of understanding the promoting effect of Nb and preventing the non-desired inert SbNbO4 phase. Two different catalyst syntheses have been tested: the most common in which Sb is added as Sb2O3 suspension and a novel one in which it is molecularly dissolved as a tartrate complex. The effect of adding Nb to alumina-supported Sb-V oxide catalysts depends on the preparation method. The preparation via Sb2O3 suspension affords segregated Sb oxide, which results in lower Sb-V interaction. The preparation via Sb tartrate complex affords molecularly dissolved Sb species that combine more easily with V or Nb species. The higher dispersion of Sb in the tartrate method also promotes its interaction with Nb dopant. The Sb tartrate preparation method is preferred for binary Sb-V supported oxide catalyst, while the Sb2O3 suspension method is preferred for Nb-promoted Sb-V supported oxide catalyst, which results in a very efficient catalyst for propane ammoxidation. (c) 2006 Published by Elsevier B.V.