HgCl2 that was homogenously dispersed over activated carbon was prepared by an HCl solution method (HgCl2(HCl)/AC), in the present study. The speciation of HgCl2(HCl)/AC in simulated flue gases was further investigated by a temperature-programmed decomposition desorption mass spectroscopy method, to clarify its transformation mechanism. It was found that HgCl2 sublimation occurred after Hg-H2O pretreatment, evidenced by the disappearance of the desorption peak of HgCl2 at 80 degrees C. After Hg-H2O pretreatment, new species of HgCl2 were formed, as confirmed by similar peak intensities of HgCl2 at mass numbers 270 and 272 at 335 degrees C. The SO2-H2O or H2S-H2O pretreatment helped stabilize the HgCl2 species supported over the AC, as evidenced by the weaker desorption peak of HgCl2 at around 80 degrees C. For the HgCl2(HCl)/AC pretreated with SO2-H2O, four desorption peaks of mercury were found, at 280, 335, 380 and 480 degrees C, indicating that new mercury compounds were formed during pretreatment or the TPDD process. The mercury desorbed at 280 degrees C was derived from a mercury complex of [HgCl](2)SO4 center dot center dot center dot A-C, while the mercury desorbed at 480 degrees C resulted from HgSO4. Some HgO species might be formed and stabilized over the AC during the TPDD process, corresponding to two mercury desorption peaks at 335 and 380 degrees C. After the pretreatment of HgCl2(HCl)/AC with H2S-H2O, some HgCl2 were transformed to HgS and HgO species, as evidenced by the mercury desorption peaks at 280, 305 and 380 degrees C.