The reaction mechanism of NO reduction by propene over silver-supported TiO2-ZrO2 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-NO2), nitrite (R-ONO), inorganic NOS, carbonate, formate and acetate species were detected when TiO2-ZrO2 or Ag/TiO2-ZrO2 was exposed to a gas mixture of NO+C3H6+O-2 at room temperature. In the case of Ag/TiO2-ZrO2, an intense IR band assigned to isocyanate (-NCO) species was observed by evacuation at temperatures above 300 degrees C after being exposed to NO+C3H6+O-2. Under a dynamic condition, the isocyanate band was not detected on Ag/TiO2-ZrO2, but observed on TiO2-ZrO2. The isocyanate species was found to be highly reactive toward NO2. A reaction mechanism has been proposed that organic nitro and nitrite compounds formed initially on TiO2-ZrO2 are converted on Ag sites to isocyanate, which is then reduced to N-2 by the reaction with NO2 On Ag sites.