The effect of oxygen concentration on the catalytic reaction of NOx with soot over BaAl2O4 has been studied by Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS). The introduction of O-2 into the NO flow can result in a reactant mixture of NO/NO2/O-2. Increasing the O-2 concentration from 2 % to 5 % in the NO-containing flow promotes the formation of NO2 from the gas phase oxidation of NO. The reactant mixture with high O-2/NO flow, allows for the formation of greater amounts of nitrate species than that with low O-2/NO flow, which further promotes the reaction of soot with NOx and leads to a high conversion efficiency of NOx into N-2 and N2O. In the absence of O-2, N2O is not observed since the N2O produced at high temperatures has reacted with soot before it can be detected.