The impact of different fly ash samples on mercury speciation in simulated flue gas at 140 degrees C was evaluated in this study. Experiments were conducted in a fixed bed reactor to determine the impact of fly ash morphological characteristics and chemical composition on mercury uptake and oxidation. No homogeneous mercury oxidation was observed at 140 degrees C. Mercury uptake tests with different fly ash samples revealed that loss on ignition (LOI), surface area, and particle size all had positive effects on mercury oxidation and adsorption (i.e., as the above parameters increased, mercury adsorption and oxidation also increased). Experiments with pure inorganic components showed that alumina (A12O3), silica (SiO2), calcium oxide (CaO), magnesium oxide (MgO), and titania (TiO2) do not promote mercury oxidation or adsorption. Ferric oxide (Fe2O3) and unburned carbon, on the other hand, showed significant mercury oxidation and capture.