The paper gives results of a study to identify active sites and surface functional groups that may contribute to the absorption of elemental mercury (Hg) by relatively inexpensive calcium (Ca)-based sorbents. The study investigated the formation of chlorine (C1) sites in CA-based sorbents as well as their role and reactivity in the absorption of Hg. HC1-exposed calcium sulfate dihydrate (gypsum) exhibited a superior Hg sorption capability. Crystalline water molecules on the surface of the gypsum were confirmed to contribute indirectly to Hg uptake. These surface molecules may have absorbed HC1 through hydrogen bond formation between an oxygen atom of a crystalline water molecule and a hydrogen atom of an HC1 molecule, Two adjacent, physically absorbed HC1 molecules could then trap an Hg molecule through formation of a mercuric-chloride-like molecule. Significant correlations between C1 and Hg uptake by gypsum were noted.