SO₂ would deactivate the low-temperature SCR (selective catalytic reduction) catalysts and reduce NO removal. In this study, Fe(0.1)−Mn(0.4)/TiO₂ prepared by sol−gel method was selected to carry out the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) investigation for revealing the mechanism of the SO₂ effect on the SCR reaction. The DRIFT spectroscopy showed that SO₂ could be adsorbed on the surface of the catalyst as the bidentate mononuclear sulfate. This type of sulfate would retard the formation of NO complex on the surface of catalyst, resulting in the decrease of NO adsorption. For NH₃ adsorption, the adsorption of SO₂ had little effect on the coordinated NH₃, but would increase the amount of NH₄⁺ because of the formation of new Brønsted acid sites. Therefore, besides the deposition of ammonium sulfates, the competitive adsorption between SO₂ and NO on the active sites of the catalysts also contributed to the poisoning effect of SO₂ on the SCR reaction. When sulfate was formed on the catalyst, much less NO could be adsorbed and take part in the SCR reaction. Copyright © 2010 American Chemical Society.