The extent of the selective catalytic reduction (SCR) of nitric Oxide to dinitrogen in the presence of excess oxygen is enhanced by the oxygen on several zeolite-based catalysts and using different reductants. When the catalyst is Cu-ZSM-5 and the reductant is a hydrocarbon, an NO2 intermediate has been suggested by several investigators. This work shows that at short residence times, with excess reductant and in the absence of oxygen, the N02 itself is reduced only back to NO. Thus, for the selective reduction of NO2 to N2 (N-pairing) strongly oxidizing conditions are required, same as for the complete reduction of NO. In the presence of excess oxygen the activity of Cu-ZSM-5 in the NO + O2 reaction to form NO2 parallels the SCR in every respect. It is higher over Cu-ZSM-5 than on Cu/Al2O3 or on H-ZSM-5. The copper-containing zeolite is also active in the decomposition of NO2 back to NO and O2 while the other catalysts are much less active. The inhibiting effect of water on the NO + O2 catalytic reaction is also parallel to the effect on SCR. This evidence strengthens the notion of an N02 inter mediate.