PURPOSE: To obtain a reverse osmosis membrane comprising separating layers of polyamide urea exerting an improved solute rejection as well as solvent flux by allowing a microporous organic polymer substrate to be impregnated with a water soluble multifunctional amine and to be treated with an isocyanate- substituted acylhalide solution.

CONSTITUTION: A microporous substrate is formed by casting a polymer solution over a support comprising non-woven or woven polyester or polypropylene fabric or the like. Polysulfones are preferable as the substrate, while the support can be of a porous inorganic material. Subsequently, dipping the substrate in a water solution of a multifunctional diame e.g. aqueous m-phenylene diamine and taking this out, excessive water is removed by a roller or the like. Then, polyamide urea is formed by dipping the substrate in a solution liquid wherein an isocyanate-substituted isophthaloylchloride, preferably 5- isocyanateisophthaloylchloride is dissolved in a solvent which is water miscible and which basically does not have adverse effects on the substrate. Thereafter, impurities are removed therefrom.