The influence of the ammonia/propylene feed ratio on the acrylonitrile/acrolein product ratio for bismuth molybdate-based catalysts indicates the presence of two mechanistic regimes. One of these occurs under normal, high turnover conditions and corresponds to a catalytic site composed of one Mo-diooxo group (the O-inserting species) which activates ammonia by formation of a Modiimido which is the active N-inserting species. The other regime at lower turnovers (by He or N 2 dilution of feed) utilizes two Mo-dioxo or -diimido species at the active site, in which one Mo species acts as an O- or N-inserting element, and the other serves as a redox element to facilitate catalyst reduction and reoxidation. In both regimes, bridging Bi 3+ -O species serve as the α-H-abstracting element to form initially a Mo-π-allyl in the rate-determining step. Similar experiments on antimonate catalysts reveal that, over the entire range of experimental conditions, active sites are composed of bridging O- (or N-) inserting atoms associated with two Sb 5+ atoms, which are bonded by a H-abstracting Sb 3+O bridge. Ammonia activation occurs by successive formation of two bridging NH groups, the N-inserting species. The isotope effects for the second H abstraction for ammoxidation over molybdate ( k H k D = 2.1 ) and antimonate ( k H / k D = 1.4) systems indicate that reversible N insertion of allyl into an unsaturated MoNH species occurs (i.e., π ⇋ σ), but a more irreversible insertion into a bridging SbNHSb species (i.e., π → σ) is favored. The isotopic distribution ( d 2 d 0 ) of acrylonitrile produced from allylic-deuterated propylene, allyl alcohol, and allylamine reveals that allylic scrambling for propylene and allyl alcohol, but CN bond retention for allyl amine, occur due to favored allyl migration from O to N, but H migration from N to O, in the molybdate surface intermediates.