The chemical vapor deposition (CVD) of some heterocyclic compounds was examined to control the porosity and surface functionality of the active carbon fiber (ACF). The deposition took place only on the pore wall of the ACF, when the heterocyclic compound as the precursor and the deposition temperature were selected carefully to be thermally stable and around 700 degrees C, respectively. Moderately activated ACFs modified with pyridine, pyrrole, and thiophene demonstrated molecular sieving activity for selective adsorptions of CO2/CH4 and O-2/N-2 through selective CVD. In contrast, furan decomposed at this temperature, failing to provide molecular sieving activity. The thermal stability of the depositing molecules is a key factor to obtain the molecular sieving performance after CVD. Pyridine, pyrrole, and thiophene produced amorphous carbon within the pore which appears to implant the nitrogen and sulfur atoms over the surface of the ACF, respectively.