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.