This study aimed at elucidating a two-step process of the low-temperature selective catalytic reduction (SCR) reaction. The data were collected through studying the EPR signal of NO (or NH3) + O-2 adsorption on fluorine-doped vanadia/titania and vanadia/titania catalysts used for SCR reaction, as well as the impact of SO2 and H2O added to the gas mixtures. The EPR experiments provided a new method to explain the SCR reaction, which were associated with NO (NH3) + O-2 reacting systems and had been inferred step by step. In summary, the first step of the two-step process was that the vanadium sites were reduced by NO or NH3 as electron donor: then, the superoxide ions appeared over vanadium. The experimental findings showed that the surface acidity, surface oxygen vacancies and the superoxide could be increased by fluorine doping, accordingly, the two-step process rate would be enhanced by F doping. It was the reason for F-doped catalyst had the higher catalytic activity than vanadia/titania. This two-step process could also be used to elucidate the inhibition mechanism of SO2 and H2O for the SCR reaction. (C) 2011 Elsevier B.V. All rights reserved.