Acid catalyzed synthesis of resorcinol-formaldehyde xerogels ends up (after carbonization) into microporous carbon materials with potential use as Carbon Molecular Sieves (CMS). CO2 activation was performed in order to modify the microporosity of the carbon xerogel. As separation of linear and branched paraffins presents an industrial relevance, 2,2′,4-trymethylpentane (TMP) and n-octane dynamic adsorption was tested in order to relate textural properties of different samples with their adsorptive behaviour. Since TMP (ramified) is bulkier than n-octane (linear), carbon xerogels with slim micropores (with low activation degree) were unable to retain this compound. Increasing the burn-off degree, adsorption of TMP is favoured by the pore width increase, but simultaneously, n-octane adsorption also increased by the development of the micropore volume. So that, competitive adsorption experiment were carried out in order to analyze the separation performance of our samples. As expected, the smaller the micropore size, the better the separation results. Therefore, a compromise is needed in order to optimize the CMS or adsorptive behaviour.