This work refers to the characterization of activated carbon obtained by chemical activation by KOH and ZnCl2 of a lignocellulosic material, by the techniques of adsorption of nitrogen at 77 K and immersion calorimetry in cyclohexane and water. The obtained results show that the enthalpy of immersion in these two liquids is in perfect correlation with the specific surface area. In addition, the oxidizing treatment by the nitric acid of the activated carbon partially transforms the mesoporosity into microporosity; indeed, the isotherms of type IV tend toward isotherms of type I; this shows that the created superficial oxygenated functions reduce the diameter of the pores preventing the accessibility of the nitrogen molecule to a part of porosity; these results are confirmed by the immersion calorimetry since the enthalpies of immersion of oxidized coals, in cyclohexane and water, decrease and increase, respectively.