Wet flue gas desulfurization (FGD) systems designed to remove sulfur dioxide can capture soluble oxidized mercury which then lead to mercury accumulation in FGD gypsum. In this study, the mercury species in three FGD gypsum samples were first determined using a temperature programmed decomposition method. The mercury species in-the FGD gypsum was mainly HgS. Mercury vapor release to the atmosphere from FGD gypsum was then investigated. The effects of temperature, UV radiation and solid moisture content on mercury release were studied in controlled laboratory experiments. Detectable amounts of mercury were constantly emitted from three samples. The mercury release-rates in the dark at room temperature were found to vary from 0.31% to 2.65% during the 180-day period tests. The temperature, UV radiation and solid moisture content all significantly enhanced the mercury release from the FGD gypsum with synergistic enhancements existing between the temperature/UV radiation and the solid moisture content. The promotion of oxidized mercury reduction and Hg-0 desorption and diffusion might be the reasons for the enhancement of mercury release by these factors. Considering the temperature, UV radiation and solid moisture content fluctuations of FGD gypsum when exposed to natural environment, the mercury release from FGD gypsum disposed outdoor might be significant. (C) 2016 Elsevier B.V. All rights reserved.