Gas-phase reactions between elemental mercury and chlorine are a possible pathway to producing oxidized mercury species such as mercuric chloride in combustion systems. This study examines the effects of quench rate, NO, SO2, HCl, and the chemistry of the sample conditioning system on apparent and actual levels of oxidation in a methane-fired (293 W or 1000 Btu/h), quartz-lined reactor. The sample conditioning system included two impingers in parallel: one containing an aqueous solution of KCl to trap HgCl2, and one containing an aqueous solution of SnCl2 to reduce HgCl2 to elemental mercury (Hg0). Gas-phase concentrations of Cl2 as low as 1.5 ppmv were sufficient to oxidize a significant fraction of the elemental mercury in the KCl impinger via hypochlorite ion. The addition of 0.5% sodium thiosulfate to the KCl solution completely prevented the impinger oxidation from occurring. The measured homogeneous mercury oxidation levels with the sodium thiosulfate additive ranged from 2 to 8% at reactor chlorine levels of 100 to 500 ppmv (as HCl). This is in contrast to oxidation levels approaching 95% in the absence of sodium thiosulfate. Quench rates ranging from 440 to 210 K/s had little effect on the extent of oxidation. Extents of oxidation were also insensitive to SO2 and NO at concentrations of 400 and 500 ppmv. The addition of thiosulfate did not inhibit the KCl impinger's ability to capture HgCl2 and the effectiveness was not affected by NO or SO2.