The reduction of NO with propene and propane on Ni-exchanged mordenite was investigated by in situ infrared spectroscopy. Nitrates and organic nitro compounds were formed on the catalyst upon exposure to NO, O-2, propene and/or propane. In the presence of propene, carbonaceous deposits consisting of oxygenated carbon species, condensed aromatic compounds and polyenes were formed. The concentration of deposits on the catalyst was considerably lower for propane. Isocyanates associated with Ni were identified as reactive intermediates during the reduction of NO to N-2. Oxygenated carbon species contribute to the formation of CO2 and might be involved in the formation of isocyanates. (C) 2003 Elsevier B.V. All rights reserved.