The free volume sizes and distributions and multilayer structures in asymmetric thin layer polyamide membranes in the wet state have been First time measured using the variable monoenergy slow position beam (VMSPB) This paper presents a newly developed method which combines the technique of plasma-enhanced chemical vapor deposition to form a barrier layer for high-vacuum condition to seal the composite polyamide and of interfacial polymerization between 2-aminoethanethiol (AETH) and trimesoyl chloride (TMC) on the surface of modified asymmetric polyacrylonitrile (mPAN) membrane (AETH-Tmc/mPAN), under the wet condition encountered during the pervaporation process in which they are in direct contact with the liquid environment provided by the feed solution During the VMSPB experiments under a high vacuum, our new method maintained the thin-film composite (TFC) membrane in the wet conditions typically experienced in VMSPB experiments wherein it directly contacts the liquid feed environment, which is essential in the course real measurements as there is a high inter action between the feed solution and the membrane The results of free volumes and layer structures in the wet state are compared with the dry sate and arc supported by the results using SEM and FTIT-ATR measurements