||The mercury in coal is emitted in its elemental state when the coal is burned. As the combustion flue gas cools, reactions under homogeneous and heterogeneous conditions between mercury and species such chlorine, bromine, SOx and NOx can take place. The temperature and concentration of these species determines the extent of mercury oxidation. The main objective of this study was to evaluate the effects of bromine, chlorine, SOx and NOx on gas-phase mercury oxidation reactions in flue gas. This study used a methane-fired, bench-scale reactor and CHEMKIN software for performing kinetic calculations. The model included reaction pathways to account for halogen, mercury, and combustion chemistry. The experimental results showed that chlorine is not effective as a gas-phase oxidant of mercury compared with other halogens such as bromine. In presence of bromine, a significant extent of mercury oxidation was observed at the two quench rates evaluated in this study. Extents of oxidation in the presence of bromine decreased when SO2 was present. A wet conditioning system was used to determine mercury speciation in oxidized and elemental species. The total side of the system reduces oxidized mercury back to elemental using a stannous chloride-hydrochloric acid solution. The elemental side contains a solution of potassium chloride-sodium thiosulfate which retains oxidized species and does not interact with the elemental mercury species. A solution of sodium hydroxide was installed in both sides of the conditioning system to remove acid species before they reach the mercury analyzer. The solutions on both sides of the conditioning system, the total and the elemental, were modified to decrease biases in measurements when working with bromine. Replacing the combustion air with a mixture of 27%O2 in CO2 caused a large increase in the apparent extent of oxidation by chlorine, increasing it from 2% to 70% at 100 ppm Cl (as HCl). Oxygen firing had little impact on oxidation by bromine. The presence of 500 ppm SO2 caused a dramatic decrease in oxidation, particularly for chlorine.