The mechanism of acrylonitrile (AN) decomposition, that is, conversion of the nitrogen atoms in AN molecules to N-2, over Cu-ZSM-5 was investigated. Oxygen was found to be necessary for AN decomposition and H2O in the reactant gas enhanced the reaction between AN and O-2. The dependence of AN decomposition on contact time indicated that HCN, HNCO, NH3, and NO, behaved like intermediates. Temperature-programmed surface reaction of adsorbed AN revealed that H2O suppressed desorption of AN and lowered the temperature of N-2 evolution. In situ diffuse reflectance infrared Fourier transform spectroscopy revealed that AN was mostly converted to isocyanate (-NCO), which was probably converted to adsorbed NH3 by hydrolysis. N-2 was formed by the reaction of adsorbed NH3 and adsorbed nitrate or by oxidation of adsorbed NH3. (c) 2007 Elsevier B.V. All rights reserved.