New insight in the mechanism of the so-called "Fast SCR" reaction over a commercial Fe zeolite catalyst was gained by an in situ FTIR investigation of the evolution of the surface intermediates during step response runs addressing the adsorption of NO2, as well as the reactivity of surface NOx with NH3 and with NO. The collected data, combined with the analysis of companion transient flow reactor experiments demonstrating the corresponding dynamics of gaseous reactants and products, confirm the key role of ferric nitrites/nitrates in the reaction pathway, and emphasize the cooperative interaction between the zeolite acidic sites, where ammonia is stored, and the Fe red-ox sites of the catalyst, which are involved in the critical reduction of surface nitrates by NO. (C) 2011 Elsevier B.V. All rights reserved.