In this study a thermal desorption procedure (HgTPD) was used to identify mercury species in samples of gypsum obtained from wet flue gas desulphurization plants (WFGD). Gypsum from industrial coal combustion power plants and gypsum from a laboratory device that simulates mercury retention in the WFGD process were studied. It was concluded that mercury sulphide (HgS) is the mercury species present in WFGD gypsums unless an additive is used. Mercury speciation in this kind of residue can contribute to a better understanding of the reaction and adsorption behaviour of mercury species in the WFGD process and provide a deeper knowledge of the environmental impact caused by the disposal or reuse of these Hg-containing residues. (C) 2015 Elsevier Ltd. All rights reserved.