The pore size and pore size distribution of carbon molecular sieve (CMS) membrane were modified by self-assisted deposition of carbon segment to enhance the separation performance of a gas pair. The poly(p-phenylene oxide) (PPO) casting dope was selected as the carbon segment-forming agent, coating the surface of the carbon membrane, followed by carbonization at 600 degrees C to form a modified carbon layer. The effect of the pore structure of the original carbon layer derived from polyimide (PI) and polyetherimide (PEI) was studied to examine the control of their pore sizes by the method proposed in the present paper. Due to intrinsic properties of low viscosity and less thermal stability, the PPO-casting dope can spread homogeneously and form a dense coke to shrink the pore size and shift the pore size distribution close to the dimension of smaller gas molecules. Thermogravimetric analysis, atomic force microscopy (AFM), field-emission scanning electron microscopy, and N-2 isotherm adsorption analysis were employed to characterize the thermal stability, roughness, and microstructural changes in the modified CMS membranes. After modification, the H-2 permeability increased from 565 +/- 10.5 to 1448 +/- 56.9 Barrer [1 Barrer =1 x 10(-1) cm(3)(STP)cm/(cm(2) s cm Hg)j, and the ideal H-2/N-2 selectivity increased from 17.1 +/- 0.6 to 171.8 +/- 7.7, respectively. The PPO/PI-derived CMS membranes exhibit potential use in CO2 capture, whereas the PPO/PEI-derived CMS membranes exhibit excellent potential for hydrogen separation in natural gas. (C) 2011 Elsevier B.V. All rights reserved.