Kinetic and in situ infrared measurements were utilized in the investigation of the selective catalytic reduction (SCR) of nitric oxide (NO) by propylene (C3H6) over a Pt/SiO2 catalyst. The results of these studies indicate the presence of two kinetically distinct regions at temperatures above and below the temperature of maximum NO reduction. They further suggest that the activation of the hydrocarbon by molecular oxygen is the kinetically significant step in the low temperature region. Similar results were previously obtained with a Pt/Al2O3 catalyst, suggesting that the mechanism of the reaction is the same in both cases. The in situ infrared spectra of these two catalysts indicate the presence of surface isocyanate and cyanide,groups in both cases, suggesting that one or both of these species may be involved in the reaction.