Due to favorable interactions between Polyvinyl chloride (PVC) and Thermoplastic Polyurethane (TPU) chain segments as revealed by viscosity measurement and differential scanning calorimetry (DSC), the ternary phase diagram of the PVC/TPU blend (for all compositions) was almost close to that of neat PVC. According to a simple diffusion model used for studying the kinetics of membrane formation, the PVC/TPU membrane structure was expected to be almost similar to that of PVC membrane which was supported by scanning electron microscope (SEM) micrographs. PVC membranes were impermeable to water up to a transmembrane pressure of 8 bar. Interestingly, adding TPU to PVC lowered the pressure at which the membrane showed permeability. Addition of 25 wt% TPU to PVC led to a substantial increase of pure water flux from 3.4 to 111.4 L/m(2) h along with no change in the separation performance (100% Coomassie Brilliant Blue (CBB) dye rejection). However, further increase of TPU content beyond 25 wt%, caused a remarkable loss in membrane performance where under a specific threshold pressure the pure water flux increased steadily with time. This phenomenon being reversible below and above the threshold pressure was ascribed to the high elasticity of TPU, justified by mechanical evaluation of membranes and their glass transition temperature.