Polyamide thin film composite reverse osmosis membranes on porous poly-sulfone support were prepared using m-phenylenediamine, trimesoyl chloride and dimethyl sulfoxide (DMSO) with various contents as precursors via an interfacial polymerization. The membrane with heterogeneous multilayer structure was depth-profiled by positron annihilation lifetime spectroscopy based on a pulsed slow positron beam and the thickness of the polyamide active layer can be estimated through fitting the results of depth dependence of o-Ps intensity. The free volume characteristics in the polyamide selective layer of the membranes were evaluated and were correlated with the permeation properties. With increasing DMSO content, mean free volume size (V-FV) was enlarged, which was responsible for the enhanced water and sodium chloride permeation. However, the salt rejection was significantly decreased with a slight increase in the V-FV around a critical value of 88.0 angstrom(3), which was the average size of hydrated sodium and chloride ions. It was well interpreted for that the broadened free volume distribution enables an amount of larger free volumes to be involved in the salt permeation, although little change in V-FV was found.