Density functional theory calculations were performed to investigate the effect of SO2 on mercury adsorption on a carbonaceous surface. The carbonaceous surface is represented by a six-fused benzene ring model in which the edge atoms on the upper side are unsaturated to simulate the active sites. All of the possible approaches in which SO2 was adsorbed on the edge sites of the carbonaceous surface were considered to examine their effects on Hg adsorption. The results indicated that SO2 adsorptions on the carbonaceous surface with OO approach and OS approach are thermally favorable. The effects of SO2 on Hg adsorption are complicated, which depends on the SO2 concentration in flue gas. The adsorption of SO2 increases the adsorption capacity of the carbonaceous surface for Hg by increasing the activity of its neighbor sites. However, higher concentrations of SO2 decrease the Hg adsorption capacity since SO2 competes for the activate sites on the carbonaceous surface. (C) 2012 Elsevier B.V. All rights reserved.