Material properties and the pore structure of activated carbon fibres of four different degrees of activation have been investigated. It is shown that there exist five main pore sizes for all degrees of activation. Narrow pores disappear upon further activation. Lower degrees of activation exhibit a greater equilibrium adsorption capacity at low solvent concentrations due to the large portion of narrow pores. This effect more pronounced in a humid atmosphere. Consequently the enthalpy of adsorption of low degrees of pore filling according to the ALI(c)-Model is much higher for lower degrees of activation than it is for higher ones.