Bench-flow reactor and in situ DRIFTS experiments were carried out to elucidate the features of the propylene + NO + O-2 reaction system on Cu-SSZI3 (chabazite) monolithic catalyst. Experiments were conducted under both steady state and transient conditions for application-relevant feed conditions. Steady state conversion data of the C3H6 light-off in the presence of excess O-2 (5%) shows inhibition by a much smaller amount of NO (500 ppm). Corresponding data in the presence of NO2 reveals a transition in the C3H6 light-off curve; for temperatures below 350 C, the C3H6 conversion is higher in the presence of NO2 as compared to 02 whereas above 350 C the C3H6 conversion in the absence of NO2 is higher. The reduction of NO2 to NO and N-2 is enhanced by the addition of C3H6, which facilitates the reduction of Cu sites favorable for the NO2 reduction. Spatially resolved concentration profiles using a series of monolith pieces of different lengths provide insight into the reaction pathways. The spatial profiles show that NO2 is completely reduced in the front part of monolith at temperatures above 350 degrees C. On the other hand, NO that is formed from the NO2 reduction increases along the length in the temperature range of 200-350 degrees C and exhibits a maximum value above 350 degrees C. Similarly, C3H6 oxidation is inhibited by NO at the light-off temperature of the C3H6 oxidation reaction (similar to 350 degrees C) while there is no effect of NO on C3H6 oxidation reaction below the light-off temperature. The results clearly suggest that inhibition is not due to a competitive adsorption process but due to the generation of partially oxidized hydrocarbon species that react with NO to form O- and N-containing surface intermediates which serve to block the active catalytic sites. In situ DRIFTS measurements confirm the formation and existence of -NCO like species over catalyst surface on exposure to NO after a 45 min of exposure to C3H6 and O-2. A phenomenological reaction mechanism is proposed that involves the formation of inhibiting intermediates through the reaction of NO with oxygenates, which then react further to form N-2.