Although C 2 H 5 OH and CH 3 OCH 3 are isomeric compounds, C 2 H 5 OH is much more efficient for NO x reduction than CH 3 OCH 3 . In situ diffuse reflectance infrared Fourier transform spectroscopy was used to study the reaction mechanism. The enolic species derived from the partial oxidation of C 2 H 5 OH over Ag/Al 2 O 3 plays a crucial role in the formation of -NCO species during the selective catalytic reduction (SCR) of NO x by C 2 H 5 OH. The high reactivity of the enolic species results in high surface concentration of -NCO species and high efficiency of NO x reduction when C 2 H 5 OH was used as the reductant. In the case of CH 3 OCH 3 , the formate species as the main intermediate has low activity for the formation of -NCO species, which results in the relatively low efficiency during the SCR of NO x .