In this paper, we describe the investigation of chemical vapor deposition (CVD) of carbon from benzene and cyclohexane on a particular pitch-based active carbon fiber (ACF) at different deposition temperatures as a means of controlling the pore size of ACF in order to induce true molecular sieving capability and to discuss the mechanism of carbon deposition. This study showed that the specific temperature range 700-800 degrees C was very effective. In this range, deposition of amorphous carbon was restricted to the pore wall and became saturated at the level of 11% weight increase, as further deposition stopped. This treatment greatly enhanced the molecular sieve separation of CH4 from CO2. Reduction of pore size appeared to be limited by the thickness of the benzene plane, since no further carbon deposit took place when the pore could no longer accept benzene. Higher temperatures allowed deposition to occur on the outer surface of ACF, which plugged the pore completely on extended reaction time. Cyclohexane was found to be inferior to benzene as a carbon precursor, as it decomposed too rapidly in the gas phase at the temperature range used and precipitated the carbon plugging the pores. Thus, high molecular sieving selectivity was not obtained.