Because mercury enters the food chain primarily through atmospheric deposition, exposure models require accurate information about mercury emission rates and mercury speciation from point sources. Since coal-fired power plants represent a significant fraction of the anthropogenic emissions of mercury into the atmosphere, the speciation of mercury in coal-fired power plant flue gas is currently an active topic of research. We have demonstrated that the assumption of gas-phase equilibrium for mercury-containing species in coal-fired power plant exhaust is not valid at temperatures below approximately 800 K (500 degrees C). Chlorine-containing species have been shown to be the most important for oxidation of elemental mercury in the post-combustion gases. The conversion of HCl to Cl-2 in the flue gas of a coal-fired power plant is kinetically limited. Kinetic calculations of the homogeneous oxidation of elemental mercury by chlorine-containing species were carried out using global reactions from the literature. The levels of mercury oxidation, while of comparable magnitude to field observations, are still below the 40% to 80% oxidation typically observed in field measurements. (C) 2000 Elsevier Science B.V. All rights reserved.