Activated carbon has been used as adsorbent for removing pollutants, e.g., VOC. However, adsorption of VOC on activated carbon is hampered by moisture in air. Thus, equilibrium isotherms of water vapor on microporous carbons containing oxygen functional groups were investigated to see the correlation between amounts of water adsorbed and site concentrations of surface oxygen functional groups. Amounts of water adsorbed depended on the amount of CO2 producing groups at low relative pressure, regardless of pore structure of the microporous carbons. This is an abstract of a paper presented at the 2007 AIChE Annual Meeting (Salt Lake City, UT 11/4-9/2007).