A commercial zeolite-based catalyst for NOx removal via ammonia SCR was studied with either pure NO or pure NO2 (as NOx compounds). The SCR efficiency was greatly enhanced when using pure NO2. The CO conversion levels enabled to illustrate that NO2 favours the CO oxidation into CO2, while NO formed would participate in the fast SCR path (like a 'masked' fast SCR process). The formation of NH4NO3 at low temperature, when using pure NO2, and its deposit onto the catalyst was evidenced by mean of the IR operando methodology. Its accumulation leads to a loss of activity for the NOx reduction, which must certainly be associated to an obstruction of the microporous system. Nevertheless, after a single thermal regeneration of the catalyst (which would arise during the catalyst 'real life' upon any increase of the engine regime), the SCR efficiency was fully recovered. (C) 2011 Elsevier B.V. All rights reserved.