This study focuses on the mechanism of lean NO(2) reduction by hydrocarbons (propane, propene, and isobutane) over HZSM-5. In-situ FTIR measurements indicate a close correlation between formation of isocyanate species, consumption of water (formed in the reaction), and formation of amine species. The results in this investigation confirm our previously suggested reaction mechanism, which involves reaction of NO(+) species and hydrocarbon-derived species over Brønsted acid sites, forming isocyanate species. These species are hydrolyzed by water, forming amine species and, finally, N(2). Experiments with (18)O(2) show an enhanced oxidation of propane by oxygen, in the presence of NO(2). This effect can possibly be explained by a type of reaction mechanism where gas-phase and/or loosely bound NO(2) react with the adsorbed hydrocarbon-derived species (i.e., carbenium ion adsorbates and/or alkenes), which then more easily react with oxygen.