The structure-transport properties of mixed soft-segmented poly(urethane-imide) (MSPUI) membranes and their microstructures were investigated. Polypropylene glycol, polycaprolactone diol and bis(3-aminopropyl)-terminated polydimethylsiloxane were used as the soft segments in the membrane synthesis via a three-step polymerization reaction. The chemical structures of the MSPUI membranes were characterized using attenuated total reflectance Fourier transform infrared spectroscopy. Morphology and surface properties of the membranes were studied using scanning electron and atomic force microscopy techniques. Surface energy measurements indicated the enrichment of the hydrophobic soft segment in the membranes. The amorphous nature of the polymers was analysed using wide-angle X-ray diffraction. The effect of morphology on the permeability and selectivity of the membranes is discussed. Finally, membrane structure-transport property relationships were correlated.