The paper gives results of a bench-scale study of the effects of flue gas and fly ash parameters on the oxidation of elemental mercury in simulated flue gases containing hydrogen chloride (HCl), nitric oxide (NO), nitrogen dioxide (NO2), sulfur dioxide (SO2), and water vapor (H2O). Gas-phase studies indicated that elemental mercury, in the presence of these gaseous species, oxidizes very slowly at low to moderate temperatures (25-350 C). In the presence of HCl, elemental mercury oxidation proceeds at measurable rates only at high temperatures (>700 C) and high HCl concentrations (>200 ppm). SO2 and H20 in the simulated flue gas significantly inhibits the gas-phase oxidation of elemental mercury in the presence of HCl. The effects of fly ash composition were investigated using a fixed bed containing model fly ashes consisting of mixtures of major components found in coal fly ashes. Work to date has focused on the potential catalytic oxidation of elemental mercury in a temperature range of 150-250 C by two- and three-component model fly ashes composed of mixtures of alumina, silica, ferric oxide, cupric oxide, and calcium oxide.