Positron annihilation spectroscopy (PAS) Coupled with a slow positron beam has been used to Study the free-volume depth profile (0-10 mu m) in polymeric membrane systems prepared by the interfacial polymerization method. Doppler broadening energy parameters of annihilation radiation vary as a function of the depth from the Surface through multi-layers in polyamide asymmetric membranes prepared under different experimental parameters, Such as the time, the temperature, and pH of membrane preparation process of interfacial polymerization. Three layer structures including a skin, a transition, and the porous layers in those asymmetric membrane systems are obtained. Positron annihilation lifetime results provide additional information about free-volume size and distribution in nano- and micro-scale film membrane systems. PAS results are correlated with the pervaporation separation, flux and water wt% in permeate of 70% isopropanol aqueous solution and compared with the SEM images.