Two different flue gas desulfurization (FGD) ash samples were exposed to a simulated flue gas stream containing elemental mercury vapor to evaluate the interactions and determine the effects of gas components, dry FGD ash samples, and temperature on adsorption and heterogeneous oxidation of mercury. Both samples were characterized for surface area, unburned carbon content, element content, and mineralogical composition. Mercury speciation downstream from the sample was determined using Ontario Hydro Method. Results showed that higher levels of mercury oxidation were associated with higher levels of mercury capture. The NO2, HCl, and Cl-2 promoted mercury oxidation, while SO2 and NO had inhibitory effects on mercury oxidation. Unburned carbon of dry FGD ash sample played an important role in mercury capture. Whether the surface area was caused by unburned carbon or by calcium-based sorbents might be more significant than the level of surface area. Extent of mercury oxidation and capture increased slightly and then decreased as the temperature rising due to the interaction of mass transfer and reaction rates control. (C) 2011 Elsevier B.V. All rights reserved.