Reactivity of surface isocyanate (NCO(a)) species with NO, O-2 and NO+O-2 in selective reduction of NOx over Ag/Al2O3 and Al2O3 catalysts was studied by a pulse reaction technique and an in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The NCO(a) species on Ag/Al2O3 reacted with O-2 or NO + O-2 mixture gas to produce N-2 effectively above 200 degrees C, while the reaction of NCO(a) with NO hardly produced N-2 even at 350 degrees C. In the case of Al2O3 alone, less N-2 was detected in the reaction of NCO(a) with NO + O-2, indicating that silver plays an important role in the N-2 formation from NCO(a). These behaviors of the reactivity of NCO(a) species with reactant gases were in good agreement with the changes in NCO(a) bands shown by in situ DRIFT measurements. Based on these findings, the role of NCO(a) species in the selective reduction of NOx on Ag/Al2O3 and Al2O3 catalysts is discussed.