Solution-phase modification techniques are conventionally used to post-treat hollow fiber membranes for enhancing the separation performance. The main limitations of these techniques are considerable time consumption and probable deformation of the membrane structure in addition to producing a large amount of solvent waste. Hence, in this study we have devised a batch method and a continuous method for vapor-phase modification. We aim to use these new modification methods to improve the H(2)/CO(2) separation of hollow fibers made from different polyimide materials whilst overcoming the drawbacks of solution phase modification. Using various surface characterization techniques, the conversion of imide to amide groups is confirmed to be predominantly at the outermost layer of the fibers. The chemical modification results in the formation of a radial dense layer at the shell side of the fibers. The H(2)/CO(2) selectivity of the polyimide/polyethersulfone dual layer hollow fiber increases from 1.7 to 35.5 with an optimal vapor-phase modification duration of 5 min. Both batch and continuous vapor phase modifications produce comparable separation performance. Since the continuous vapor-phase modification method can significantly simplify the post-treatment process and reduce solvent waste, it has great potential for the scale up and industrialization of the effective process of using diamine modification to modify hollow fiber membranes for hydrogen purification. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.