The paper gives results of an investigation conducted to characterize and modify mercury speciation in Illinois coal combustion flue gas so that a mercury control strategy can be implemented in conventional flue gas desulfurization (FGD) units. Mercury, in trace concentration in coal, is readily volatilized during coal combustion and leaves the high-temperature zone as gas-phase elemental mercury (Hgo). Previous results have shown that, as the flue gas is cooled, a portion of the Hgo is converted, primarily to mercuric chloride (HgC12), in the presence of catalytically active surfaces such as certain fly ashes. Unlike Hgo, HgC12 is highly soluble in water and has a high affinity for alkaline sorbents; it can, therefore, be easily removed in wet scrubbers and spray dryers (FGD units). This two-task study examined the effect of injecting Illinois coal combustion residues (ICCRs) on the conversion of Hgo to HgC12 in coal combustion flue gases. Task I studied Hgo oxidation using a fixed-bed, bench-scale reactor in a simulated Illinois coal combustion environment.