The in situ treatment of reverse osmosis (RO) membranes of Loeb-Sourirajan type (asymmetric cellulose acetate (CA) membranes) by means of aqueous acetone solutions was studied by different techniques (permeations, FFIR spectroscopy and atomic force microscopy). It appears that the skin layer of the asymmetric membranes had pores of molecular size which make them much less selective in pervaporation (PV) than the dense membranes made of the same materials. A mechanism was proposed for the "defects" reparation of the membrane skin layer by this method of post-treatment. Under the action of a solvent medium that increases sufficiently the polymer chain mobility, the reparation of chains at the boundary of the polymer nodules in the active-layer seals the inter-nodule gaps (i.e. the defects). The active-layer healing is sufficient for the "tight" asymmetric RO membranes to be quite selective in PV (C) 2003 Elsevier Science B.V. All rights reserved.