Treatment of sugar charcoal, irrespective of the history of its formation, with concentrated nitric acid results in the fixation of an appreciable amount of oxygen, most of which on high-temperature evacuation is given off as carbon dioxide and has been termed as CO2-complex. The treatment results in o-considerable increase of surface acidity which is almost equivalent to the CO2-complex formed on the surface. This offers some insight into the mechanism of base adsorption and cation-exchange capacity of charcoal. The treatment also results in making the surface hydrophilic as it increases greatly the moisture-sorption capacity throughout the entire range of vapour pressure. The increase at higher vapour pressures appears to be due to the formation of new micropores and that at lower vapour pressures due to the interaction of water vapour and active stites provided by the CO2-complex by a mechanism involving hydrogen bonding. The heat of immersion in water also increases considerably, the increase on an average being about 4 kcal. per mole or CO2-complex, which is of the same order as the heat of dissolution of CO2 in water.