Carbon molecular sieves (CMSs) for separating gaseous mixtures have been prepared through chemical vapour deposition (CVD) of benzene on activated carbon fibres (ACFs) obtained from Nomex aramid fibres. The effect of the CVD treatment on the porous texture of the ACFs has been followed by immersion calorimetry into liquids of different molecular sizes (dichloromethane, benzene and cyclohexane). Both the kinetics of the immersion process and the equilibrium immersion enthalpies have been studied. Besides, the derived specific surface areas accessible to the different liquids employed have been calculated. The results show that the carbon deposition has taken place mainly at the pore entrances and the CVD treatment has succeeded to introduce selectivity without a significant loss of capacity. (C) 2004 Elsevier B.V. All rights reserved.