Abstract: The selective catalytic reduction (SCR) of NO by CH₄ in the presence of SO₂ was investigated over the catalyst cobalt supported on Mordenite (Co/MOR); both the fresh and spent catalysts were characterized by temperature-programmed desorption of NO (NO-TPD), X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR), and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The results indicated that the presence of SO₂ in the feed leads to a decrease of NO conversion at a temperature below 550℃; however, this influence is not evident at a temperature above 600℃. Moreover, the negative effect of SO₂ on the NO conversion is reversible over the Co/MOR catalyst. SO₂ may inhibit the adsorption of NO on the active sites and promote the formation of CoO during the reaction, which leads to a decrease of the active sites and the activity of Co/MOR for the SCR of NO.