To identify and quantify active Fe species in the selective catalytic reduction (SCR) of NO by NH3, a series of Fe/ZSM-5 catalysts was prepared by impregnation, reductive solid-state ion exchange, and chemical vapor deposition (CVD) under different levels of Fe loading, and was characterized by XRD, temperature-programmed desorption (TPD) of NH3 and NO2. and UV-vis, Mossbauer, and FT-IR spectroscopies. Their SCR activity depended strongly on the preparation method, and CVD was the most effective technique. Three types of Fe species were coexisted: aggregated alpha-Fe2O3 particles, FexOy oligomer, and ion-exchanged oxo-Fe3+. Relative concentration of oxo-Fe3+ as determined by T-O-T perturbation peak in FT-IR correlated with the higher temperature peak in NO2-TPD, which was linearly related to the catalytic performance. Thus, oxo-Fe3+ sites were considered to be the active sites and were accurately quantified using NO2-TPD. Furthermore, the turnover frequency was independent of the preparation method and Fe loading. (C) 2008 Elsevier Inc. All rights reserved.