Reverse selective membranes comprising poly(ethylene oxide) (PEO) containing copolyimides (PEO-PI) with variations of acid dianhydrides and diamines have been synthesized for hydrogen purification. The reverse selectivity of the membranes decimate the energy required for hydrogen recompression process. Factors including PEO content, PEO molecular weight, and fractional free volume (FFV) that would affect the gas transport performance have been investigated and elucidated in terms of degree of crystallinity, phase separation in the PEO domain as well as inter-penetration between the hard and soft segments. In mixed gas tests of CO2 and H-2 mixtures, a highly condensable CO2 out compete H-2 for the sorption sites in hard segment and diminishes H-2 permeability. Thus the CO2/H-2 selectivity in the mixed gas tests is much higher than that in pure gas tests. Mixed gas permeation tests at 35 degrees C and 2atm show that the best reverse selective membranes have a CO2 permeability of 179.3 Barrers and a CO2/H-2 permselectivity of 22.7. The physical properties of PEO-PIs have also been characterized by FTIR, DSC, GPC, WAXS, AFM and tensile strain tests. (C) 2010 Elsevier Ltd. All rights reserved.