A new type of membrane composed of a microfiltration substrate and a pore-filling polyelectrolyte has been produced by UV-induced grafting of 4-vinylpyridine onto polyethylene and polypropylene microfiltration membranes. By imposing the rigid structure of the polyolefinic substrate on the highly charged but extremely flexible and swellable,eel of polyelectrolyte, stable membranes with very high charge density (>3.5 mequiv/g) have been developed. The membranes show an outstanding pH valve effect and the capability of rejecting small inorganic ions in the process of reverse osmosis. The pressure-driven transport and separation with these membranes differ markedly from those of conventional reverse osmosis membranes being governed by the properties of the polyelectrolyte. Salt rejection-depends strongly on the feed concentration, decreasing substantially with increase in concentration. The most characteristic features of separation with these membranes are higher rejection of polyvalent co-ions and lower rejection of polyvalent counter-ions compared to monovalent ions.