In this study, the effect of the structure of polyether-based polyurethane (PU) membranes on their gas separation properties has been investigated. In this regard, a series of polyurethanes based on hexamethylene diisocyanate (HDI) and 1,4-butanediol as hard segments and different soft segments such as poly(tetramethylene glycol) (PTMG, 2000 g/mol), poly(ethylene glycol) (PEG, 2000 g/mol) and PTMG/PEG mixture were synthesized. PU membranes were prepared by thermal phase inversion method and their physical properties were analyzed by FT-IR, XRD and DSC analyses. The obtained results from FT-IR and DSC analyses indicated that the phase separation of hard and soft segments decreased and chain mobility was restricted, resulting in the increase in glass-transition temperature of the soft segment, by content of the PEG in the PU structure. The permeability of pure N-2, O-2, CH4 and CO2 gases were determined using constant pressure method at different feed pressures and temperatures. The results of gas permeation experiments showed that by increasing the ether group content in the polymer structure, permeability of the pure gases decreased, while CO2/N-2 ideal selectivity increased. The obtained results also indicated that the permeability of CO2 decrease from 132.52 barrer in PU0 (PU containing 100 wt% of PTMG in soft segment) to 20 barrer in PU100 (the PU containing 100 wt% of PEG in soft segment), respectively. CO2/N-2 selectivity increased from 28 to 90. Trade-off evaluation also showed that the potential of studied membranes for commercialization in CO2/N-2 and even CO2/CH4 separations increase by PEG content in polymer and the PU membrane which contains 75/25 wt% ratio of PEG/PTMG had the highest. (c) 2012 Elsevier B.V. All rights reserved.