An investigation of speciated mercury transformation with the addition of hydrogen bromide (HBr) at elevated temperatures was conducted in a slipstream reactor with real flue gas atmospheres. A real flue gas atmosphere is composed of bituminous coal (with high sulfur and high chlorine contents) and Powder River Basin (PRB) coal (with lower sulfur and chlorine contents). The average sulfur, chlorine, and mercury contents in the tested bituminous coal were 1.31% and 1328 and 0.11 ppm and 0.61% and 170 and 0.08 ppm, respectively, for tested PRB coal. The average CaO, Fe2O3, and loss on ignition in collected fly ash contents of tested bituminous coal were 1.71, 17.5 1, and 7.13% and 22.95, 4.9 1, and 0.64%, respectively, for tested PRB coal. The different contents of coal chlorine, CaO, and Fe2O3 in fly ash can be attributed to the different mercury speciations at baseline tests for these two coals in this study. The addition of HBr concentrations into the flue gas was controlled in the 3-15 ppm range. Semi-continuous mercury emission monitors were used to check the variation of mercury speciation at sampling locations. The Ontario Hydro Method (ASTM D6784-02) was used for data validation or comparison. For both methods, a high temperature inertial sampling probe was used to minimize the interference between vapor phase mercury and fly ash. Its temperatures were controlled consistently with flue gas temperatures at their installation locations in the slipstream reactor. Test results indicated that adding HBr into the flue gas at several parts per million strongly impacted the mercury oxidation and adsorption, which were dependent upon temperature ranges. Higher temperatures (in the range of 300-350 degrees C) promoted mercury oxidation by HBr addition but did not promote mercury adsorption. Lower temperatures (in a range of 150-200 degrees C) enhanced mercury adsorption on the fly ash by adding HBr. Test results also verified effects of flue gas atmospheres on the mercury oxidation by the addition of HBr, which included concentrations of chlorine and sulfur in the flue gas. Chlorine species seemed to be involved in the competition with bromine species in the mercury oxidation process. With the addition of HBr at 3 ppm at a temperature of about 330 degrees C, the additional mercury oxidation could be reached by about 55% in a flue gas atmosphere by burning PRB coal in the flue gas and by about 20% in a flue gas by burning bituminous coal. These are both greater than the maximum gaseous HgBr2 percentage in the flue gas (35% for PRB coal and 5% for bituminous coal) by thermodynamic equilibrium analysis predictions under the same conditions. This disagreement may indicate a greater complexity of mercury oxidation mechanisms by the addition of HBr. It is possible that bromine species promote activated chlorine species generation in the flue gas, where the kinetics of elemental mercury oxidation were enhanced. However, SO2 in the flue gas may involve the consumption of the available activated chlorine species. Thus, the higher mercury oxidation rate by adding bromine under the flue gas by burning PRB coal may be associated with its lower SO2 concentration in the flue gas.