Active carbons were treated with oxidising agents: 10% or 21% oxygen in nitrogen or solutions of concentrated HNO3 or potassium permanganate with or without the addition of sulfuric acid. The number and type of surface oxygen functionalities were investigated by potentiometric titration. Proton binding isotherms and the distribution of surface groups with pK(a) between 3 and 10 were calculated. The comparison of potentiometric titration, water vapour adsorption and thermal decomposition experiments offered the method to estimate the number of very acidic groups with pK(a) lower than 3. They are suggested to be carboxylic groups in the nearest vicinity of other species (such as e.g. NO2 groups).