Carbon molecular sieves (CMS) are valuable materials for the separation and purification of gas mixtures. Their ability for selective adsorption has been exploited commercially in the Pressure Swing Adsorption process. Activated carbon fibers, are suitable materials for the production of CMS, due to their narrow pore size distribution and their ability to adsorb and desorb gases rapidly. A pore size tailoring technique, by plasma deposition, was used aiming to the formation of pore constrictions, by narrowing the pore system of commercial activated carbon fibers. In this work hydrocarbon RF plasma was employed to enhance propylene or ethylene cracking on the fibers surface. For this reason propylene/nitrogen or ethylene/nitrogen discharges of 80 and 120W were used. The molecular sieving properties of samples were evaluated by measuring the adsorption of CO2 and CH4, under ambient conditions, and by calculating the ideal selectivities. Chemical modifications of the ACFs surface, produced after plasma treatment, were evaluated by XPS analysis, while SEM was used to examine the fiber surface before and after plasma treatment. Results obtained show that the pore tailoring by RF plasma was effective, since ideal selectivities of CO2 and CH4 were significantly improved. The relative improvement was found to depend on the gas used for the deposition process. XPS analysis proved that nitrogen reacts and remains on the sample surface during plasma treatment.