Abstract: A semi-permeable asymmetrical membrane, useful for water desalting by reve osmosis and comprising cellulose acetate polymer, is formed from a casting solution in which there is incorporated, as a swelling medium, organophilic bentonite, that is, bentonite (preferably Montmorillonite) which has been directly reacted with di-methyl-di-stearyl (quaternary) ammonium chloride to have had its exchangeable cations replaced by long-chain hydrocarbons. The bentonite is swelled in dioxane, and the solution also includes acetone, equal parts of cellulose diacetate and cellulose triacetate, acetic acid and methanol. The steps of casting, cold water immersion and annealing are generally conventional. Incorporation of the bentonite results in high salt restraining capacity along with high product water flow and good flow stability.