During combustion of pulverized coals, most of the mercury (Hg) in coal volatilizes. With the wide use of dry and wet scrubber systems for flue gas desulfurization (FGD), various amounts of Hg are captured by coal combustion byproducts, such as, FGD gypsum. The specification of Hg in FGD gypsum is essential not only to determine the risk when the wastes are recycled or disposed but also to understand the behavior of Hg during coal combustion and the mechanisms of Hg oxidation along the flue gas path. In this study, a temperature-programmed decomposition technique was used in order to acquire an understanding of the Hg species associated with FGD gypsum. A series of Hg reference compounds were used to obtain the characteristic temperature of decomposition for each Hg compound. The decomposition temperature of the Hg species is in an increasing order as Hg2SO4 < Hg2Cl2 < HgCl2 < black HgS < HgO < red HgS < HgSO4. The results also indicate that HgCl2 and HgS are the primary Hg compounds in FGD gypsum samples. Hg stability during the reutilization of FGD gypsum is important due to its health and environmental impact, and its value in improving the Hg emission inventory. The industrial calcining process of wallboard production is simulated in order to determine the Hg emission percentage during the reutilization of FGD gypsum. Results indicate that 12.1-55.1% of total Hg would be emitted from FGD gypsum in this process. A larger percentage of Hg will be emitted when the proportion of Hg2Cl2 and HgCl2 is higher. The Hg re-emissions from FGD gypsum during wallboard production in China were estimated to be 4.7 tons in 2008. (C) 2013 Elsevier Ltd. All rights reserved.