This study focuses on the mechanism of lean NO2 reduction by propane in the presence of water, over an acidic zeolite (HZSM-5). Fourier Transform Infrared spectroscopy measurements with NO2 and propane in excess oxygen show formation of surface bound NO+, isocyanate, unsaturated hydrocarbons and traces of amine species. Upon addition of water the isocyanate species disappear and amine species are formed. Hence, it seems likely that the isocyanate species are hydrolysed to amine species, which are possible reaction intermediates in the HC-SCR reaction over HZSM-5.