Silver/alumina is one of the most promising catalyst materials for HC-SCR. The reaction mechanism remains incompletely understood, however. Isocyanate species (-NCO) and cyanide species (-CN), important intermediates in the reaction path in HC-SCR, were investigated by gas-phase FTIR and DRIFT spectroscopy in step-response experiments. In accordance with the literature, the corresponding gas-phase species, HNCO and HCN, were always detected at the same time at the different steps of the experiment; however, the accumulation and consumption of surface -NCO and -CN species did not show a similar correlation. Consequently, in conflict with the literature, we propose two parallel, independent reaction pathways for -NCO and -CN species. In one of these reaction pathways, surface nitrates react with C-containing surface species either directly to -NCO or via R-NO2 species. In the other pathway, C-containing surface species react with gas-phase NOx, or short-lived N-containing surface species to R-NO species and subsequently to -CN species. (c) 2008 Elsevier Inc. All rights reserved.