In situ DRIFT spectroscopy and mass spectrometry techniques were employed to investigate the mechanisms of the selective catalytic reduction of NO with NH3 on the FeW oxides catalysts. By integrating the adsorption peaks of NH3 or NOx, it was found that the coordinated NH3 and NH4+ were the main active NH3 ad-species, while the adsorbed NOx was only obviously detected below 250 degrees C. That was to say that the reaction mechanisms were totally different at different temperature regions. At the low temperature region (< 250 degrees C), the reaction path was likely to be a "fast NO/NO2-SCR" process, involving the formation of NO2, complexes NO2(NH3)(2) and NO2(NH4+)(2) as intermediates. These active complexes could react with gaseous NO to produce N-2 and H2O. The oxidation of NO to NO2 was probably the rate-determining step for the SCR reaction. At the high temperature region (>= 250 degrees C), NH2NO species formed from the reaction between surface coordinated NH3/NH4+ species and gaseous NO were the key intermediates, which were further reduced to produce N-2 and H2O.