Cross-linked urethane/urea membranes with two soft segments, polypropylene oxide and more hydrophobic polybutadiene, were prepared by extending a polypropylene oxide-based tri-isocyanate-terminated prepolymer (PU) with polybutadiene diol (PBDO). The membranes obtained were analysed with relation to the structure of phase-separation, degree of cross-linking and gas permeation. The PU/PBDO membranes showed different degrees of cross-linking and different degrees of phase separation according to the variation of the ratio of PU and PBDO. It was found that the CO2 permeability of the membranes is dependent on the feed pressure and the ratio of PU/PBDO. A 67 wt.% PBDO membrane has a lower degree of cross-linking and displays a linear behaviour of CO2 permeability vs. pressure, ranging from 90 to 550 barrer. A 20 wt.% PBDO membrane has a higher degree of cross-linking and displays phase separation, leading to higher CO2 permeability, ranging from 150 to 950 barrer.