The selective catalytic reductions of NOx over Cu-ZSM-5 and Co-ZSM-5 catalysts were carried out by using CH₄ as a reducing agent. NO reacted with O₂ at room temperature to produce NO₂, N₂O₄, N₂O, and nitrate species. It was confirmed that nitrate species reacted with the decomposed reducing agent to produce N₂, CO₂ and H₂O etc. In situ FTIR spectroscopy has been to observe the species deposited on Cu-ZSM-5 and Co-ZSM-5 during the decomposition of nitromethane at 300 ℃. This is accompanied by the formation of bands due to NCO species between 2150 and 2300 cm¹ and the sudden development of a very strong band at 1660 cm¹. the latter band can be assigned to the s-triazine compound, melamine, and/or derivatives of it, which cause deactivation by blocking zeolite channels.