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 O-2 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 O-2 and CO2 and it was non-hemolytic and less thrombogenic.