A method for modifying the transfer characteristics of a porous organic or inorganic membrane, wherein (a) at least one layer is formed on the membrane by applying at least one uniform solution prepared by mixing one or more rare earth or alkaline-earth fluoroalkoxides with an anhydrous organic solvent at room temperature and in an inert atmosphere; (b) hydrolysis is performed by contacting the layer of (a) with an amount of water at least as great as the stoichiometric amount needed to hydrolyse the fluoroalkoxides; and (c) rinsing the membrane with water to remove the resulting soluble salts. The resulting modified membranes are useful for regenerating photographic solutions, particularly black and white developers, and for separating organic compounds from aqueous effluents.