Catalysts from three different catalytic systems, Ni-Nb-O, Mo-V-Nb-Te-O and Sb-V-O, have been prepared, characterized, and tested during both ethane and propane ammoxidation reactions, in order to obtain acetonitrile and acrylonitrile, respectively. The catalytic results show that Mo-V-Nb-Te-O and Sb-V-O catalyze propane ammoxidation but are inactive for ethane ammoxidation whereas Ni-Nb-O catalysts catalyze both, ethane and propane ammoxidation. The activity results, and the characterization of fresh and used catalysts along with some data from previous studies, indicate that the ammoxidation reaction mechanism that occurs in these catalytic systems is different. In the case of Mo-V-Nb-Te-O and Sb-V-O, two active sites appear to be involved. In the case of Ni-Nb-O catalysts, only one site seems to be involved, which underlines that the mechanism is different and take place via a different intermediate. These catalysts activate the methyl groups in ethane, on the contrary, neither ethane nor ethylene appear to adsorb on the Mo-V-Nb-Te-O and Sb-V-O active sites. [GRAPHICS] .