In this study, vapor-phase ethylenediamine (EDA) is utilized to specifically modify the physicochemical properties of the outer surface of polyimide membranes without modifying the internal membrane structure for hydrogen purification. The surfaces of polyimide membranes before and after EDA-vapor modification have been characterized by Fourier transform infrared-attenuated total reflectance (FTIR-ATR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), which confirmed the modification mechanism including the conversion of imide groups into amide groups with simultaneous cross-linking between polymer chains and a physical decrement in d-space. Based on pure gas permeation tests, only a 10-min vapor-phase EDA treatment can significantly improve H(2)/CO(2) selectivity (up to similar to 100). This is attributed to intensive surface modification by EDA vapor, hence rendering this simple and yet novel technique more effectively for hydrogen purification than the conventional solution approach. Although the H2/CO2 separation performance in mixed gas tests is not as superior as that in pure gas permeation tests, mixed gas results affirmed impressive H(2)/CO(2) separation performance of vapor-phase EDA modified polyimide membranes. This novel vapor modification strategy appears to be promising for large-scale processes, especially the modification of hollow fiber membranes for industrial hydrogen purification. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.