The effect of SO3 on elemental mercury adsorption on a carbonaceous surface is investigated by the density-functional theory calculations. A nine-fused benzene ring model is employed to represent the carbonaceous surface. The edge atoms on the upper side of the model remain unsaturated to simulate the active sites for reaction. All of the possible approaches in which SO3 is adsorbed on the carbonaceous surface are conducted to evaluate their effects on Hg adsorption. The results indicate that the carbonaceous surface is energetically favorable for SO3 adsorption, which causes that SO3 competes for the active sites on the carbonaceous surface. But adsorption of SO3 decreases the adsorption capacity of the carbonaceous surface for Hg-0 since SO3 suppresses the activity of its next-nearest-neighbor carbon atom and negatively affects on the frontier molecular orbitals and LUMO-HOMO energy gap of the carbonaceous surface. (C) 2012 Elsevier B. V. All rights reserved.