The synthesis of two-soft-segment urethane/ urea polymeric membranes with various proportions of the two soft segments, poly(propylene oxide) and polybutadiene, yielded very distinct morphologies depending on the degree of phase segregation. The morphologies were identified with transmission electron microscopy. With a low concentration of polybutadiene, this soft segment segregated into ellipsoidal microdomains dispersed in a poly(propylene oxide) matrix. With an intermediate concentration of polybutadiene, the morphology was characterized by nanoscale phase separation and could be described as a disordered, wormlike domain structure. With a high concentration of polybutadiene, a single phase was observed. This was attributed to molecular mixing of the two soft segments and was associated with membranes that under the application of a shear stress developed bands that efficiently scattered light. These bands were identified by field emission scanning electron microscopy with a periodicity of approximately 4-5 mu m. The change in the membrane morphology from microscale phase segregation to nanoscale phase segregation led to very different membrane gas-permeation properties, that is, a reduction of the CO2 permeability from 191 to 90 Barrer. (C) 2006 Wiley Periodicals, Inc. J Appl Polyrn Sci 103: 315-320, 20071031315320