Ultrathin membranes for separating substances are obtained by exposing shaped articles comprising a water-soluble polymer or a mixture of a water-soluble polymer and water-insoluble polymer to a plasma of a gas selected from helium, argon, nitrogen, hydrogen, oxygen, carbon monoxide, carbon dioxide, ammonia, a hydrocarbon having 1 to 10 carbon atoms, an epoxide having 2 to 10 carbon atoms, or an alkylamine, or a mixture of these substances to crosslink the surface layer, and then washing the exposed articles with water to remove the unreacted portions. The membrane exhibits outstanding performance characteristics when used in the processes for separating or concentrating substances, such as reverse osmosis, ultrafiltration, and the like.