The selective catalytic reduction (SCR) of NOx in excess of oxygen using ethanol as reducing agent on silver/alumina was studied by coupling in situ FTIR spectroscopy and mass spectrometry. A comparison with the same reaction on bare alumina permitted us to show the connection between the presence of silver and isocyanate species on a catalyst. Then, a detailed investigation concerning these groups was undertaken to understand their formation, their localization, and their reactivity in order to propose the pathway of the NOx SCR into N-2. Three elemental sequences are suggested explaining, first, the formation of silver cyanide and its transformation into Al3+NCO, then isocyanates hydrolysis into ammonia, and finally the reaction of the latter species with NO in the presence of oxygen giving rise to nitrogen. (C) 2003 Elsevier Science (USA). All rights reserved.