Reactor activity measurements and FTIR spectroscopy were applied to study the transient and steady state behavior of Cu and Fe exchanged zeolite catalysts for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with NH(3) at 200 degrees C. Different surface NOx adsorption complexes were observed on the two catalysts. IR bands assigned to surface nitrate/nitrite groups were apparent on the Cu catalyst under most testing conditions. On the Fe catalyst, IR bands assigned to surface nitro groups were also observed, in addition to nitrate/nitrite groups. Both the nitrate/nitrite and the nitro group showed high reactivity with NH(3), which explained similar initial transient SCR activity of the two catalysts. The presence of NH(3) was found to block the Fe sites from catalyzing the oxidation of NO and severely inhibited the overall SCR activity when there was no NO(2) in the gas mixture. Addition of NO(2) improved the low temperature SCR activity of both catalysts. The higher SCR activity on the Fe catalyst at the optimum NO:NO(2) = 1:1 was attributed to the high intrinsic activity of the nitro groups formed on the Fe catalyst. (C) 2010 Elsevier B.V. All rights reserved.