Effect of SO2 on elemental mercury (Hg-0) oxidation over a highly active CeO2-TiO2 catalyst was systematically investigated. SO2 was found to have different, even contrary, effects on Hg-0 oxidation under different flue gas conditions. In pure N-2 atmosphere, SO2 inhibited Hg-0 oxidation. In N-2 plus O-2 atmosphere, low concentration of SO2 promoted Hg-0 oxidation, while high concentration of SO2 deteriorated Hg-0 oxidation. The promotional effect of SO2 on Hg-0 oxidation was probably due to SO3 generated from SO2 oxidation, and the inhibitive effect of SO2 on Hg-0 oxidation was attributed to the competitive adsorption between SO2 and Hg-0. The results suggest a balance point for SO2 concentration, where the promotional effect on Hg-0 oxidation by SO3 originated from SO2 is equal to the inhibitive effect of SO2 on Hg-0 adsorption and subsequent oxidation via the Langmuir-Hinshelwood mechanism. In the presence of NO, SO2 with the aid of O-2 exhibited promotional effect on Hg-0 oxidation through NO-catalyzed oxidation to form SO3 and hence a balance point of higher SO2 concentration. However, without O-2, SO2 greatly limited Hg-0 oxidation in the presence of NO. When HCl was present, most Hg-0 oxidation was due to reactions between active chlorine species and adsorbed Hg-0. SO2 inhibited Hg-0 adsorption and therefore deteriorated Hg-0 oxidation by chlorine species. When both NO and HCl were present, NO accelerated the conversion of SO2 to SO3, and hence relieved the prohibitive effect of SO2 on Hg-0 oxidation by HCl. (C) 2013 Elsevier B.V. All rights reserved.