Positron annihilation spectroscopy (PAS) has been used to study the depth profile (0 - 8 µm) in membrane systems containing a thin layer of polyamide prepared by the interfacial polymerization of triethyl-enetetraamine (TETA) and trimesoyl chloride (TMC) onto the surface of asymmetric polyacrylnitrile (PAN). Doppler broadening S parameters of annihilation radiation energy spectra show a systematic variation with the doping time of TETA in the membrane preparation process of interfacial polymeriza-tion. The layer structures and free-volume properties of composite membrane are obtained. A similar variation between S parameters with respect to TETA doping time and permeability but an anti-correlation between S parameter and selectivity are observed. Applications of PAS and slow posi-tron beam to membrane technology will be discussed.