Mo-V-Nb-Te-oxide catalysts with nominal metal compositions equal to those of M1, M2, and a mixture of the M1 and M2 phases were prepared in neat and sol-diluted forms. The latter catalysts were prepared with 30 wt% of Al2O3, SiO2, or TiO2 sol. All catalysts were characterized by BET, XRD, and TPR and tested for the respective ammoxidation of propane and propene. XRD showed that dilution with SiO2 gave catalysts with the pure M1 and M2 phases. Al2O3 was found to react with Mo and V, giving catalysts with Al-2(MoO4)(3), AlVO4, TeMo5O16, Nb2O5, and, in one case, M2. Dilution of the nominal M1 composition with TiO2 produced Mo5-x(V/Nb/Te)(x)O-14, whereas the M2 and M1/M2 nominal compositions gave the correct type of phases. TPR and XRD reveal that dilution with SiO2 gives improved dispersion of M1 and M2; however, the dilution effect is greater, resulting in no improved activity when expressed per unit surface area of the catalyst. For propane ammoxidation, the activity data show that the neat M1 and M1/M2 catalysts are the best samples. The corresponding SiO2-diluted catalysts are less active and selective. For the ammoxidation of propene, the neat M1 and the SiO2-diluted M1 are the two best catalysts in terms of both activity and selectivity. More selective catalysts with lower activity and some acrolein formation are the neat M2 phase and M2 diluted with TiO2 or SiO2. (c) 2006 Elsevier Inc. All rights reserved.