The paper gives the result of a study that focused on the oxidation of elemental mercury (Hgo) in the presence of both complex, four-component, model fly ashes and actual coal combustion fly ashes collected from a pilot plant. Steady state oxidation of Hgo promoted by the four-component model fly ash containing calcium oxide, CaO, was reached at slower rates, compared with the three-component model fly ash with no CaO as studied previously. The partial removal of gas phase hydrogen chloride (HCl) by CaO in the CaO-containing model fly ashes reduced the available chlorinating agent, resulting in slower oxidation of Hgo. All the coal fly ashes were more active in the oxidation of Hgo in the presence of nitrogen oxides (NOx), compared to HCl. The results agree with those of the previous model fly ash studies. The product of heterogeneous reaction of Hgo with NOx was identified by other researchers as mercuric nitrate monohydrate, which is volatile and stable under flue gas cleaning conditions. Sodium oxide and nickel may also be active in oxidizing Hgo.