Microcalorimetry was employed to study the surface chemistry of activated carbon. The chemistry of active sites was found to be a function of the pretreatment procedure. Following treatment at high temperature (950 degrees C) in nitrogen, the carbon adsorbed a great deal of oxygen at 25 degrees C with a remarkably high heat of adsorption (approx. 125 kcal/gmol O-2), whereas after treatment at high temperature in hydrogen the same carbon adsorbed virtually no oxygen at 25 degrees C. It was found, however, that the hydrogen treated carbon did adsorb oxygen at an elevated temperature (150 degrees C). Changes in the character of oxygen interaction with the carbon surface can be related to the nature of unsaturated sites on the carbon surface. Specifically, after treatment in nitrogen the surface contains a high concentration of highly unsaturated ('dangling') carbon atoms which interact rapidly and strongly with oxygen. All these sites are removed by the hydrogen treatment leaving only partially unsaturated sites, which only interact with oxygen at elevated temperature.