The occurrence of elemental mercury in flue gases from coal combustion is a problem of current environmental concern. Oxidized mercury species can be effectively removed from the flue gases by chemical scrubbers. However, the detailed mechanism by which oxidation occurs remains unclear. Theoretical rate constants are calculated for mercury oxidation by atomic chlorine. The potential energy surface is determined using standard quantum chemical methods with relativistic effects included via the use of an effective core potential (ECP). Experimental thermodynamic and kinetic data are employed to assess the accuracy of these calculations. Results show that the QCISD method with the 1992 basis set of Stevens et al. gives good agreement with experiment, suggesting that this combination may be useful for other mercury-chlorine chemical systems. (C) 2003 Elsevier B.V. All rights reserved.