Stabilities and/or decomposition characteristics of mercury compounds are important for the design of solid sorbents for mercury vapor removal from coal-derived flue gas and fuel gas. However, available data on the stability and decomposition behavior of mercury compounds are inadequate. The stability and/or decomposition behavior of mercury compounds, such as HgS (metacinnabar and cinnabar), HgO, HgSO 4 , HgCl 2 , and Hg 2 Cl 2 , were investigated by the temperature-programmed decomposition and desorption technique using a mass spectrometer (TPDD-mass) method. The effects of solid diluents, such as quartz, SiC, Al 2 O 3 , TiO 2 , or activated carbon (AC), on the decomposition characteristics were also studied by the TPDD-mass method. In particular, the stability and reactivity of mercury chloride (HgCl 2 ) in coal combustion flue gas and coal-derived fuel gas conditions were examined. The following results were obtained: (1) the order of the main peak temperature of mercury evolution from the decomposition of the mercury compound diluted with quartz sand in He flow was as follows: HgS (metacinnabar) = HgO < HgS (cinnabar) < HgSO 4 ; (2) HgSO 4 was hydrolyzed with H 2 O; (3) HgO was reduced by SO 2 in the presence of H 2 O and O 2 ; (4) HgCl 2 and Hg 2 Cl 2 over SiO 2 were more easily decomposed than the other mercury compounds; (5) Among the diluents of HgCl 2 , SiO 2 , SiC, Al 2 O 3 , TiO 2 , and AC, HgCl 2 was most easily decomposed to Hg 0 over SiO 2 ; (6) AC as a diluent apparently stabilizes HgCl 2 ; and (7) HgCl 2 gas could be converted to Hg 0 over quartz wool, Pyrex wool, ceramic (SiO 2 -Al 2 O 3 ) wool, carbon fiber, and AC at high temperatures (>ca. 200°C).