Langmuir—Blodgett multilayers of polymerized diin amphiphiles on different porous, hydrophobic ultrafilters have been studied as composite membranes under the conditions of reverse osmosis. Influences of differently structured surfaces of the supports on structure and phase change occurring during polymerization have been investigated by scanning electron microscopy and X-ray diffraction and are discussed with reverse osmosis data. When conditioned, fine porous hydrophobic supports become permeable to aqueous solutions at low pressures. Under these conditions permeate flux and retention were found to depend on the number of layers deposited.