Hydrogen cyanide is formed over Cu/ZSM-5 during the selective catalytic reduction of NO(x) by either propene or ethene in the temperature range 450-600 K. Under the reaction conditions used (reactant feed : 973 ppm NO, 907 ppm propene or 1448 ppm ethene, 2% oxygen, W/F= 0.1 g s cm-3), the concentration of hydrogen cyanide reaches 20, respectively, 30 ppm, depending on whether ethene or propene are used as hydrocarbons. In addition, significant N2O formation is observed at temperatures lower than 700 K, independent of the hydrocarbon used.