The mechanism of selective catalytic reduction (SCR) of NOx with NH3 over Fe/MFI was studied using in situ FTIR spectroscopy. Exposing Fe/MFI first to NH3 then to flowing NO + O-2 or using the reversed sequence, invariably leads to the formation of ammonium nitrite, NH4NO2. In situ FTIR results in flowing NO + NH3 + O-2 at different temperatures show that NH3 is strongly adsorbed and reacts with impinging NOx. The intensity of the NH4NO2 bands initially increases with temperature, but passes through a maximum at 120 degreesC because the nitrite decomposes to N-2 + H2O. The mechanistic model rationalizes that the consumption ratio of NO and NH3 is close to unity and that the effect of water vapor depends on the reaction temperature. At high temperature H2O enhances the rate because it is needed to form NH4NO2. At low temperature, when adsorbed H2O is abundant it lowers the rate because it competes with NOx for adsorption sites.