The effect of chlorine on mercury oxidation and nitrogen oxides (NO x ) reduction over selective catalytic reduction (SCR) catalysts was investigated in this study. Commercial SCR catalysts achieved a high Hg 0 oxidation efficiency when Cl 2 was sprayed into the flue gas. Results indicated that an appropriate concentration of Cl 2 was found to promote NO x reduction and Hg 0 oxidation significantly. An optimal concentration of Cl 2 (25 ppm) was found to significantly promote NO x reduction and Hg 0 oxidation. Moreover, we studied the effects of Cl 2 on NO x reduction and Hg 0 oxidation over SCR catalysts under different concentrations of SO 2 . The SO 2 poisoning effect was decreased by Cl 2 when the SO 2 concentration was low (below 1500 ppm). However, sulfate gradually covered the catalyst surface over time during the reaction, which limited the impact of Cl 2 . Finally, different sulfur-poisoned catalysts were examined in the presence of Cl 2 . The NO x reduction and Hg 0 oxidation performances of sulfate-poisoned catalysts improved when Cl 2 was added to the flue gas. Mechanisms for NO x reduction and Hg 0 oxidation over fresh catalysts and sulfate-poisoned catalysts in the presence of Cl 2 were proposed in this study. The mechanism of Cl 2 -influenced NO x reduction was similar to that for the NH 3 -SCR process. With Cl 2 in the flue gas, the number of Brønsted active sites increased, which improved catalytic activity. Furthermore, Cl 2 reoxidized V 4+ –OH to V 5+ =O and caused the NH 3 -SCR process to operate continuously. The Langmuir–Hinshelwood mechanism was followed for Hg 0 oxidation by SCR catalysts when Cl 2 was in the flue gas. Cl 2 increased the number of Lewis active sites, and catalytic activity increased. Hg 0 adsorbed on the surface of the catalysts and was then oxidized to HgCl 2 . Adding Cl 2 to the flue gas increased the strength and number of acid sites on sulfate-poisoned catalysts.