Oxidized mercury species may be formed in combustion systems through gas-phase reactions between elemental mercury and halogens, such as chorine or bromine. This study examines how bromine and chlorine species affect mercury oxidation in the gas phase. Experiments were conducted in a bench-scale, laminar, methane-fired (300 W), quartz-lined reactor, in which gas composition (HCl, HBr, NOx, and SO2) was varied. In the experiments, the postcombustion gases were quenched from the flame temperature to about 350 degrees C and then speciated mercury was measured using a wet conditioning system and continuous emission monitor (CEM). Bromine was shown to be much more effective in the postflame, homogeneous oxidation of mercury than chlorine, on an equivalent molar basis. The addition of NO to the flame (up to 400 ppmv) had no impact on mercury oxidation by chlorine or bromine. The addition of SO2 had no effect on mercury oxidation by chlorine at SO2 concentrations below about 400 ppmv; some increase in mercury oxidation was observed at SO2 concentrations of 400 ppmv and higher. The addition of chlorine caused minor increases in the extent of oxidation by bromine. The results of this study can be used to understand the relative importance of gas-phase mercury oxidation by bromine in combustion systems.