Active carbons of different origins have been oxidized with H2O2 and (NH4)2S2O8 and their oxygen surface complexes have been characterized by TPD, classical titration following Boehm's method and by neutralization calorimetry. The net enthalpies of neutralization, determined by immersion calorimetry into NaOH and HCl 2 N lead to −41.1±1.8 and −52.3±2.0 kJ eq−1 for the acidic and basic sites on the surface. Experiments with NaHCO3 lead to −39.7±1.7 kJ eq−1 for the carboxylic groups alone. These results suggest that the surface groups of active carbons can also be characterized by immersion calorimetry. Results are also given for the variation of the pH of the point of zero charge with the total oxygen content of the surface.