The reactivity of surface adsorbed species over Na-H-mordenite has been examined by using dynamic in situ IR spectroscopy and the mechanism of the selective reduction of NO with C3H6 discussed. When Na-H-mordenite was exposed to a flow of NO-C3H6-O-2 at 573 K, a strong absorption band assignable to NO3- was observed at 1394 cm(-1). The intensity of the band significantly decreased when the flowing gas was switched to C3H6-O-2. The rate of decrease in NO3- was ca.3.0 x 10(-8) mol g(-1) s(-1) which was equivalent to the rate of NO conversion to N-2 in the NO-C3H6-O-2 flow reaction. Thus, it was indicated that NO3- is a reaction intermediate for the formation of N-2 and the surface reaction of NO3- is the rate determining step of the formation of N-2 in the selective reduction of NO with C3H6. When the flowing gas was switched to C3H6, the rate of decrease in NO3- species was twice as fast as that in C3H6-O-2, i.e., the reactivity of NO3- species strongly depended on the presence of O-2. Since NO was also formed simultaneously with N-2 in the case of NO2-C3H6 reaction, it was suggested that NO3- species oxidize C3H6 accompanying not only the conversion into N-2 but also the dissociative decomposition into NO. Weak bands assignable to adsorbed hydrocarbon, carbonyl, nitrite and isocyanate were also observed. On the basis of the changes in the intensity of these bands, the consecutive reaction of surface organic species with NO3- species over Na-H-mordenite was suggested.