The sorption of mercury, as Hg-0 and HgCl2, in a synthetic flue gas (SFG) by three activated carbons has been examined by XAFS spectroscopy. The three carbons consisted of a sulfur-activated carbon, an iodine-activated carbon, and an activated carbon derived from lignite. In addition to mercury, the occurrence and behavior of sulfur, chlorine, calcium, and iodine were also examined by XAFS spectroscopy. These other elements were present either as activating species on the carbons or as reactive components (SO2, HCl) in the SFG. The XAFS results showed that each type of activated carbon behaves differently with respect to sorption of mercury and other species from the SFG. For the iodine- and sulfur-activated carbons, the XAFS data confirm that it is the activating element (I or S) that forms a sorption complex with mercury. However, the activated carbon from lignite exhibited a more variable behavior that reflected the conditions of the experiments, in particular whether HCl or HgCl2 was present in the SFG. This study reveals some of the complexities that are involved in low-temperature sorption of mercury by activated carbons in that the sorption mechanism clearly involves acidic species of sulfur and chlorine in the gas phase, the affinity of the carbon for such species, and the nature of the active sites on the carbons, in addition to the mercury speciation.