A vacuum pulse method (Temporal-Analysis-of-Products (TAP) reactor) and in situ FTIR were applied to study the mechanism of the reduction of NO2 to N-2, by C3H6 over an acidic H-mordenite catalyst. An adsorbed nitrile is formed on the catalyst surface presumably via a nitro or nitroso compound of propene. Upon reaction with water the nitrile yields adsorbed ammonium ions. Ammonium ions on the surface form nitrogen upon reaction with NO2 and may therefore be considered as the decisive intermediate of NO2 reduction by propylene over acidic mordenite catalysts.