Gas-phase mercury oxidation by halogens under air- and oxygen-fired (27% O-2 in CO2) conditions was studied in a bench-scale, laminar, methane-fired, 300 W, quartz-lined reactor. The experiments provided data on the extents of elemental mercury oxidation in the presence of various flue gas components, including chlorine, bromine, SO2, and NO. A wet conditioning system and a Tekran 2537A mercury analyzer were used to determine mercury speciation and concentrations. Under the same experimental conditions and for both halogens, oxy-firing produced modest increases in mercury oxidation. Extents of oxidation at chlorine concentrations of 100-500 ppmv (as HCl) ranged from 6 to 21% for oxy-firing and from 4 to 15% for air-firing. Oxidation by bromine at concentrations of 10-50 ppmv (as HBr) ranged from 43 to 69% for oxygen-firing and from 15 to 46% for air-firing. Under both firing conditions, the addition of NO (below 300 ppmv) had little or no effect on mercury oxidation by chlorine or bromine and the addition of SO2 (below 400 ppmv) had no impact on oxidation by chlorine. A possible explanation for higher oxidation under oxy-firing conditions is that the CO2 molecule is a more effective third body than N-2; that is, carbon dioxide is more effective than N-2 at removing energy from the HgCl and HgBr transition-state complexes.