Abstract: A composite dynamically formed membrane is described comprising an intermate layer between a solid porous supporting material and the layer of forming particles.In forming the composite dynamic membrane an intermediate layer of inert material which is either granular or fibrous, is deposited on a substantially tubular porous supporting device and on this there is deposited a second layer of an inert material, the ratio of the particle diameters between the second and the first layer being between 1:10 and 1:1,000.The fine particles for this second layer may be blended with the particles of membrane forming material.Dynamic membranes are employed in reverse osmosis, which more in particular is widely used for the desalination of aqueous liquids.