The adsorption of elemental mercury (Hg⁰) on activated carbons modified with 0.2, 0.6, and 1 M HCl is systematically examined. Breakthrough curves are measured, and coupled adsorption and desorption experiments with temperature-programmed desorption (TPD) are performed. The experiments show that impregnation with HCl produces surface-bound chlorine, which significantly increases the capacity of activated carbons for mercury. Physisorptive interactions between elemental mercury and the activated carbon surface dominate on the basic materials. In contrast, on HCl-modified activated carbons, chemisorptive interactions of Hg⁰ with surface-bound chlorine lead to a complex involving carbon, chlorine, and mercury. Using TPD, two mechanisms could be identified that yield reaction products with different energetic values. By continuously recording Hg⁰ and Hg_total concentrations, the formation of Hg⁰ and Hg _x Cl₂ during desorption of the complexes from the surface could be studied. It is shown that Hg _x Cl₂ found in TPD is not present as a solid salt in the pores but is formed by thermal degradation of the mercury chlorine complex on the carbon surface. The mass fraction of Hg measured in TPD which is bound in Hg _x Cl₂ depends on the Hg loading of the activated carbons, with a maximum mass fraction of 27%. We propose that an important step in the chemisorptive reaction with increasing mercury loading is the conversion of a HgCl₂ complex into a Hg₂Cl₂ complex.