This is the first report indicating that the presence of Bronsted sites, acting in synergy with redox sites in Ga-modified MFI zeolites, can catalyse the ammoxidation of propane. Two catalysts with gallium loadings of 2 and 0.3 wt.% were prepared by ion-exchange of MFI zeolites with Si/Al ratios equal to 33 and 150, respectively. The integrity of the zeolites was essentially preserved following ion-exchange, as demonstrated by surface area, porosity, and crystallinity measurements. In situ C-13 and N-15 MAS NMR studies, using propane-2-C-13 and (NH3)-N-15 as labelled reactants suggest that Ga-modified H-MFI catalysts promote the formation of C-N bonds when propane is reacted in the presence of ammonia and oxygen (C3H8:NH3:O-2 molar ratio = 1:3:2. T = 673 K). Conventional microcatalytic reactor results confirm the conclusions of the in situ MAS NMR investigations. Propane is converted to mainly propene and acrylonitrile. with minimal production of COx (CO and CO2). in the temperature range 723-773 K. Increasing the temperature to 773 K worsens the selectivity to acrylonitrile, whereas increasing the molar ratio NH3:C3H8 from 2 to 3 increases the selectivity to acrylonitrile and decreases conversion. Both the conversion and the selectivity to acrylonitrile increase when the molar ratio O-2:C3H8 is increased from 0.3 (sub-stoichiometric) to 2 (stoichiometric). A new reaction pathways is proposed in which a cyclic protonated pseudo-cyclopropanr (PPCP) transition state is responsible for the low temperature activation of propane. (C) 2001 Elsevier Science B.V. All rights reserved.