Abstract: A method for producing a microporous separator for an electrochemical cell comprises dissolving a copolymer of acrylonitrile into N,N-dimethylformamide, impregnating an acid- and/or alkali-resistant fibrous web with the resultant solution, and thereafter dipping the impregnated web into an aqueous coagulating bath, whereby the copolymer deposits on and between the web fibers to form a microporous matrix. The aqueous coagulating bath is selected from the group consisting of water at a temperature of 15.degree. C or below and aqueous solutions of a water-soluble acid solute, basic solute, or netural solute at a concentration of one mol / 1 or above. This method yields products having low maximum pore radii and yet low electric resistances.