Understanding the decomposition characteristics of mercury compounds is very important for the development of sorbents for mercury removal from coal derived flue gas and fuel gas. The information concerning to the stability of mercury compound are valuable data necessary for the design of solid sorbents. However, there are not enough data available on mercury compound's stability in the flue or fuel gas conditions. The decomposition behavior of mercury compounds such as HgS, HgO, HgSO4, HgCl2 Hg2Cl 2 were investigated by TPDD (temperature programmed decomposition and desorption) technique using a mass spectrometer (TPD-Mass method). Especially, the stability and reactivity of mercury chloride (HgCl2 was mainly examined in coal combustion flue gas and coal derived fuel gas conditions. The mercury compounds were diluted with solids (diluents) such as SiO2 SiC. Al2O3 TiO2 and activated carbon for TPD-Mass experiments. The following results were obtained: (1) the order of the main peak temperature of mercury evolution from the decomposition of mercury compound in He flow was as follows: HgS(meta-cinnabar) = HgO < HgS (cinnabar) < HgSO4 (2) HgSO4 was hydrolyzed by H2O (3) HgO was reduced by SO2 in the presence of H2O and O 2; (3) Hg2Cl2 and Hg2Cl2 were decomposed more easily over the SiO2 than the other mercury compounds; (4) HgCl2 was most easily decomposed over the SiO 2 and stabilized over the AC; (5) HgCl2 gas could be converted to Hg0 over quartz wool, Pyrex wool, ceramics (SiO 2-Al2O3) wool, carbon fiber and AC.