The paper discusses the effects of iron content in coal combustion fly ashes on speciation of mercury. The study focused on the elemental mercury (Hgo) oxidation reactivity of coal fly ashes with different coal ranks and iron contents. Hgo oxidation tests over a high-iron-content bituminous coal (Blacksville) sample and a sub-bituminous coal ash with low iron content (Valmont) were conducted in the presence of more complex simulated flue gases. The effects of adding three gases-nitrogen oxides (NO x), sulfur dioxide (SO2), and moisture (H2O)-and two fly ashes on Hgo oxidation were evaluated. Test results confirmed the strong combined inhibition effect of SO2 and H2O on Hgo oxidation observed from a previous model fly ash study. Experiments examined the effect of increasing the iron content of a low-iron-content subbituminous coal fly ash and a lignite fly ash on their Hgo oxidation reactivity. After adding ferric oxide to these two samples to reach an iron content similar to that of the Blacksville fly ash, significant Hgo oxidation activity was measured for these two iron-doping samples. Hgo oxidation tests were performed on the Blacksville and Valmont samples.