The effect of varying physical and chemical properties of activated carbons on adsorption of elemental mercury (Hg-0) was studied by treating two activated carbons to modify their surface functional groups and pore structures. Heat treatment (1200 K) in nitrogen (N-2), air oxidation (693 K), and nitric acid (6N HNO3) treatment of two activated carbons (BPL, WPL) were conducted to vary their surface oxygen functional groups. Adsorption experiments of Hg-0 by the activated carbons were conducted using a fixed-bed reactor at a temperature of 398 K and under N-2 atmosphere. The pore structures of the samples were characterized by N-2 and carbon dioxide (CO2) adsorption. Temperature-programmed desorption (TPD) and base-acid titration experiments were conducted to determine the chemical characteristics of the carbon samples. Characterization of the physical and chemical properties of activated carbons in relation to their Hg-0 adsorption capacity provides important mechanistic information on Hg-0 adsorption. Results suggest that oxygen surface complexes, possibly lactone and carbonyl groups, are the active sites for Hg-0 capture. The carbons that have a lower carbon monoxide (CO)/CO2 ratio and a low phenol group concentration tend to have a higher Hg-0 adsorption capacity, suggesting that phenol groups may inhibit Hg-0 adsorption. The high Hg-0 adsorption capacity of a carbon sample is also found to be associated with a low ratio of the phenol/carbonyl groups. A possible Hg-0 adsorption mechanism, which is likely to involve an electron transfer process during Hg-0 adsorption in which the carbon surfaces may act as an electrode for Hg-0 oxidation, is also discussed. Published by Elsevier Science Ltd.