Nitrosonium ions, NO+, are formed upon reaction between NO, NO2, and the Bronsted acid sites of H-MOR. It is shown that the NO+ ions are highly reactive towards propene, forming propenal oxime at 120 degrees C. At temperatures above 170 degrees C, propenal oxime is dehydrated to acrylonitrile. A mechanism is proposed to explain the acrylonitrile formation. The nitrile can be hydrolysed to yield adsorbed ammonium ions, which are known to be efficient in reducing nitrogen oxides to nitrogen. The dehydration of propenal oxime appears to be the rate determining step in nitrile formation.