Several researchers have determined that water (H2O) and sulfur dioxide (SO2) in a flue gas stream have an impact on the amount of elemental mercury (Hg-0) that is homogeneously oxidized by a chlorine-containing species. Generally, it is concluded that H2O inhibits Hg oxidation by chlorine (Cl-2). However, doubt remains as to whether SO2 promotes or inhibits Hg oxidation. Further, most published results seem to indicate that nitric oxide (NO) does not have a significant impact on Hg oxidation. This paper will present data taken in a laboratory-scale apparatus designed to test these observations. In this work, Cl-2 is intentionally added to a synthetic flue gas stream containing known amounts of elemental mercury. This gas blend is similar to a flue gas obtained by burning Powder River Basin coal in a pulverized coal fired power plant and is subject to a time-temperature profile similar to a power plant. The results obtained show that H2O, SO2, and NO all have an inhibitory effect on the homogeneous oxidation of Hg by Cl-2. Further, the presence of H2O increases the inhibitory effect of SO2 and NO. Two new reactions are proposed to explain these results, in which SO2 and NO react with Cl-2. The consequences of these reactions are a reduction in the oxidative interactions that take place between Hg and Cl-2, thus decreasing the amount of Hg oxidation that occurs.