The identification and quantification of individual physicochemical forms of mercury (Hg) emissions from coal-fired systems is imperative for addressing questions concerning atmospheric fate and emission control. The flue gas composition and ash characteristics can have a significant impact on Hg speciation. Unfortunately, there is a lack of information available on mercury behavior under gasification conditions, which are different from combustion conditions. A bench-scale test apparatus was designed and built to simulate the synthesis gas conditions. The primary goal of this bench-scale work was to determine which gas constituents typical of gasification (CO, CO2, HCl, Cl-2, NH3, HCN, COS, and H2S) affect the oxidation of elemental mercury, which was delivered to the system via temperature-controlled permeation tubes. There appear to be a number of interactions between various synthesis gas constituents that affect mercury speciation. The results indicated that the reducing environment is not favorable for Hg oxidation via gas-phase reactions alone and that more elemental mercury is expected to remain in the syngas from coal gasification. However, depending on the temperature and concentration, there is clearly an interaction between fly ash and gas combinations to promote the mercury oxidation rate. Bench-scale tests also indicated that the chemistry of mercury is very complex. Further verification of this study has been planned at a pilot-scale entrained-flow gasifier.