Segmented polyurethane/urea membranes, containing two soft segments, poly(propylene oxide) and polydimethylsiloxane, were prepared by extending a polypropylene oxide-based tri-isocyanated prepolymer (PU) with polyclimethylsiloxane bis(hydroxyalkyl) terminated (PDMS). PU/PDMS membranes were characterized by infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and by the gel content. It was concluded that the two soft segments are phase separated and the urethane/urea groups form small aggregates somewhere in these two phases. Gas permeation experiments led to the conclusions that the permeabilities Of CO2, O-2 and N-2 increased with the increase of PDMS membrane content, while the permeability ratios, P(CO2)/P(N-2) and P(O-2)/P(N-2) did not significantly changed. The membrane with highest PDMS content and highest permeabilities is characterized by a lower degree of cross-linking and by a lower contribution of hydrogen bonding between hard segments. (c) 2005 Elsevier Ltd. All rights reserved.