The natural mode of retention of mercury (Hg) in flue gas desulfurization (FGD) gypsum used in wallboard manufacturing has been investigated using a series of phase-targeted reagents. The sequence of batch extractions, based on the geochemical literature, allowed for separation of particular mineral phases and subsequent testing for the appearance of Hg in the resulting extract. Results indicated that Hg was associated with two distinct phases. Most of the Hg was solubilized by the extraction targeting iron oxides and hydroxides. The reagent targeting the organic matter and sulfide minerals also typically yielded significant amounts of Hg. Analysis of the extract indicated the presence of another phase, possibly clay minerals, that may have been associated with the retained Hg in addition to, or instead of, the targeted phase. Hg was solubilized only under extremely acidic (pH < 1) and oxidizing conditions at high temperature. Virtually no Hg was found in the extracts produced by weaker reagents.