In this study, the effect of presence of HCI and SO2 in the simulated coal combustion flue gas on the Hg° removal by a commercial activated carbon (coconut shell AC) was investigated in a laboratory scale fixed bed reactor in temperature range of 80 to 200 °C. The characteristics (thermal stability) of the mercury species formed on the sorbents under various adsorption conditions were investigated by temperature programmed decomposition desorption (TPDD) technique. It was found that the presence of HCI and SO 2 in the flue gas affected the mercury removal efficiency of the sorbents as well as the characteristics of the mercury adsorption species. The mercury removal rate of AC increased with HCI concentration in the flue gas. In the presence of HCI and absence of SO2 during Hg° adsorption by AC, a single Hg° desorption peak at around 300 °C was observed in the TPDD spectra and intensity of this peak increased with HCI concentration during mercury adsorption. The peak at around 300 °C may be derived from the decomposition and desorption of mercury chloride species. The presence of SO2 during mercury adsorption had an adverse effect on the mercury removal by AC in the presence of HCI. In the presence of both SO2 and HCI during Hg° adsorption by AC, the major TPDD peaks temperatures changed drastically depending on the concentration of HCI and SO2 in flue gas during Hg° adsorption.