An experimental study of hetero- and homogeneous mercury oxidation chemistry was conducted in a bench-scale flame-based flow reactor with a residence time of 2.9 s. Homogeneous mercury oxidation levels increased with an increasing HCl concentration, with oxidation increasing to 29% at HCl concentrations of 555 parts per million by volume (ppmv). The presence of SO2 alone also led to Hg oxidation, with approximately 20% oxidation observed at SO2 concentrations ranging from 100 to 900 ppmv. When both HCl and SO2 were present, mercury oxidation was enhanced in the presence of SO2 when the concentration of HCl was 200 ppmv and inhibited when concentration of HCl was 555 ppmv. To examine heterogeneous effects, iron oxide or montmorillonite particles were injected into the post-flame gases of the system. Mercury oxidation by HCl was enhanced at high iron oxide particle concentrations (>100 m(alpha-Fe2O3)(2)/m(3) of flue gas) compared to the homogeneous system. When iron oxide particles were injected in the presence of 100-400 ppmv SO2 without HCl, mercury oxidation was enhanced at particle loadings less than or equal to 1 m(alpha-Fe2O3)(2)/m(3) of flue gas and decreased with an increasing particle loading. At a SO2 concentration of 500 ppmv, mercury oxidation increased with an increasing particle concentration. Without HCl or SO2 in the system, mercury oxidation in the presence of iron oxide particles alone was found to be negligible. To determine if the increased oxidation by HCl or SO2 occurred solely because of an increased surface area, montmorillonite particles were injected, and under these conditions, no increase in the extent of mercury oxidation was observed, suggesting that iron oxide particle surfaces are an important contributor to the promotion of mercury oxidation in coal combustion systems.