The reaction mechanism of NO reduction by propene over silver-supported TiO 2–ZrO 2 catalyst was studied by means of in situ FT-IR, combined with catalytic activity studies. The catalytic activity studies suggested that silver plays an important role as the active species. In FT-IR measurements under a static condition, organic nitro (R-NO 2), nitrite (R-ONO), inorganic NO − 3, carbonate, formate and acetate species were detected when TiO 2–ZrO 2 or Ag/TiO 2–ZrO 2 was exposed to a gas mixture of NO+C 3H 6+O 2 at room temperature. In the case of Ag/TiO 2–ZrO 2, an intense IR band assigned to isocyanate (–NCO) species was observed by evacuation at temperatures above 300°C after being exposed to NO+C 3H 6+O 2. Under a dynamic condition, the isocyanate band was not detected on Ag/TiO 2–ZrO 2, but observed on TiO 2–ZrO 2. The isocyanate species was found to be highly reactive toward NO 2. A reaction mechanism has been proposed that organic nitro and nitrite compounds formed initially on TiO 2–ZrO 2 are converted on Ag sites to isocyanate, which is then reduced to N 2 by the reaction with NO 2 on Ag sites.