The effect of oxygen concentration on the pulse and steady-state selective catalytic reduction (SCR) of NO with C3H6 over CuO/gamma -Al2O3 has been studied by infrared spectroscopy (IR) coupled with mass spectroscopy studies. IR studies revealed that the pulse SCR occurred via (i) the oxidation of Cu-0/Cu+ to Cu2+ by NO and Oz, (ii) the co-adsorption of NO/NO2/O-2 to produce Cu2+(NO3-)(2), and (iii) the reaction of Cu2+(NO3-)(2) With C3H6 to produce N-2, CO2, and H2O. increasing the O-2/NO ratio from 25.0 to 83.4 promotes the formation of NO2 from gas phase oxidation of NO, resulting in a reactant mixture of NO/O-2/O-2. This reactant mixture allows the formation of Cu2+(NO3-)(2) and its reaction with the C3H6 to occur at a higher rate with a higher selectivity toward N-2 than the low O-2/NO flow. Both the high and low O-2/NO steady-state SCR reactions follow the same pathway, proceeding via adsorbed C3H7-NO2, C3H7-ONO, CH3COO-, Cu-0-CN, and Cu+-NCO intermediates toward N-2, CO2, and H2O products. High O-2 concentration in the high O-2/NO SCR accelerates both the formation and destruction of adsorbates, resulting in their intensities similar to the low O-2/NO SCR at 523-698 K. High O-2 concentration in the reactant mixture resulted in a higher rate of destruction of the intermediates than low O-2 concentration at temperatures above 723 K.