AbstractPoly(ether siloxane urethane urea) with different polysiloxane contents were synthesized and characterized. The single gas permeation properties of poly(ether siloxane urethane urea) membranes were determined at 30 °C for O2, N2, CO2, CH4, n-C4H10. Their structures and gas permeation properties were compared. Significant correlation among the polysiloxane soft segment with the permeation properties was found. In addition, an analysis of the viability to use these membranes for n-C4H10/CH4 separation was made. A membrane with interesting properties such as higher n-C4H10/CH4 selectivity for a mixture of gases than for single gases was obtained, indicating a possibility of application of the poly(siloxane urethane) membranes for n-C4H10/CH4 separation.