Using activated carbon (AC) and H2O2 in aqueous solution, the influence of the method of preparation of samples on the stability of the surface state toward heating and outgassing was investigated. In the preparation of samples, AC was treated with H,O, solutions of varying concentration or pH, and containing Fe2+, Fe3+, ethanol or ether. Also, the outgassing of AC and the contact between the intermediate products and the oxidizing solutions were effected under controlled temperature and time conditions. The mass of sample increased markedly when AC was oxidized at pH 2.5, especially when outgassing at 350 degrees C or for 6 h. The variations in mass at uncontrolled pH and at pH 11.5 were only slight. In the study on the thermal behavior, the samples were heated in a thermobalance between 30 and 800 degrees C in N-2 and outgassed in a gas adsorption apparatus (150 degrees C at 133 x 10(-6) Pa, for 12 h). The results of the thermogravimetric analysis showed that the mass decrease of AC increased above 600 degrees C. The stability of the surface state was mainly influenced by pH changes in the H2O2 solution and by the outgassing time of AC. The addition of ethanol or ether to the oxidizing solution lowered the stability. The mass loss was strongly dependent on the oxidation method of AC. Outgassing of the samples significantly increased the mass loss with regard to the heat treatment carried out to the same temperature.