Experiments using activated carbon to capture elemental mercury were performed using a bench-scale fixed bed reactor and a flow reactor to determine the role of surface moisture in mercury adsorption. Three activated carbon samples, which have different pore structure and ash contents, were tested for mercury adsorption capacity. From both fixed bed reactor and flow reactor experimental results, the moisture on Hg adsorption was observed for all three samples, despite extreme differences in their ash contents, suggesting that this effect is not associated with ash content. Temperature programmed desorption (TPD) experiments performed in the carbon samples after the Hg adsorption experiments indicated that chemisorption of Hg is a dominant process over physisorption for the moisture-containing carbon samples and diminished for the heat-treated moisture-free samples. X ray absorption fine structure spectroscopy results provide evidence that mercury bonding on the carbon surfaces was associated with oxygen through a mechanism likely involving electron transfer processes.