We present herein a study performed under transient reactive conditions aimed at elucidating the mechanism of the Fast SCR reaction. Transient response data collected at low temperature over a commercial V2O5-WO3/TiO2 catalyst studying the reactivity of NH3-NO/NO2 mixtures with different NO/NOx feed ratios (from 0 to 1) resulted in the identification of a novel reaction pathway for the Fast SCR reaction, which provides a comprehensive description of the chemistry associated with both NH3-NO2 and NH3-NO/NO2 reacting systems and has been validated step by step by dedicated experiments. In summary, the first step of the Fast SCR reaction is NO2 dimerization and disproportion to nitric and nitrous acid; then, the presence of adsorbed ammonia species results in the formation of ammonium nitrite, which readily decomposes to N-2 and water, and of ammonium nitrate; this species was found to be a terminal product only in the absence of a suitable reducing agent. When NO is included in the feed, however, it can effectively reduce ammonium nitrate to ammonium nitrite, that once more decomposes to N2; thus, complete selectivity to N2 is achieved by adding NO to the NH3/NO2 mixture, the optimal expected NO/NO2 feed ratio being 1/1 as experimentally observed. (c) 2006 Elsevier B.V. All rights reserved.