Ag/Al2O3 is an active catalyst for selective catalytic reduction (SCR) of NOx using long paraffins as reducing agents. In view of the important role of surface nitrates in this catalytic chemistry, their formation on the Ag/Al2O3 catalyst, i.e. on the alumina support and on the silver species, was quantified in this study under typical reaction conditions of 500-1000 ppm NOx, 6% O-2 and 12% H2O. The NOx adsorption capacity and the stoichiometry of nitrate formation and elimination reactions were determined as function of the temperature and the composition of the NOx using volumetric determinations in a flow system. The silver chemistry was investigated using in situ DR UV-vis spectroscopy. In the presence of water, nitrate formation out of NOx proceeds according to two different mechanisms depending on the state of silver as Ag(0) or Ag(I) on the catalyst. Minute concentrations of hydrogen (3500 ppm) are responsible for Ag(I) reduction. The study reveals the origin of the promotion of the low temperature SCR-NOx activity of Ag/Al2O3 by hydrogen. Hydroxylation-dehydroxylation of the alumina surface of Ag/Al2O3 catalyst was found to be an oscillating reaction, whereas the alumina support itself did not display this phenomenon. (c) 2005 Elsevier Inc. All rights reserved.