Three granular and one fibrous activated carbon with different physicochemical characteristics were subjected to heat treatment at 900 degrees C under vacuum or hydrogen flow. Subsequently, oxygen chemi- and physisorption isotherms were volumetrically obtained at 34 degrees C. Oxygen sorption experiments showed lower amounts of oxygen uptake by the H(2)-treated than by the vacuum-treated carbons, indicating that H(2) treatment effectively stabilized the surfaces of various carbons tested in this study. At low pressures, from approximately 0.001 to approximately 5 mmHg, adsorption of oxygen was governed by irreversible chemisorption, which was well described by the Langmuir equation. At higher pressures oxygen uptake occurred as a result of physisorption, which was in agreement with Henry's law. Kinetic studies showed that oxygen chemisorption was affected by both carbon surface chemistry and porosity. The results indicated that oxygen chemisorption initially started in the mesopore region from the high energetic sites without any mass transfer limitation; thus a constant oxygen uptake rate was observed. Once the majority of these sites were utilized, chemisorption proceeded toward the less energetic sites in mesopores as well as all the sites located in micropores. As a result, an exponential decrease in the oxygen uptake rate was observed.