The structural versatility of polyurethane based membranes is enhanced through the introduction of two soft segments to obtain new membranes with improved hemocompatibility. Crosslinked polyurethane/urea membranes with two soft segments were prepared by extending a polypropylene oxide-based tri-isocyanate terminated prepolymer (PU) with polybutadiene diol (PBDO). The ratio of PU to PBDO was 100/0 and 90/10. The membranes were characterised in terms of permeation towards O2 and CO2 and their hemocompatibility was evaluated through hemolysis and thrombosis experiments. It was found that the membrane with PBDO had the highest permeability to O2 and CO2 and it was non-hemolytic and less thrombogenic.