Abstract: A hollow fiber is provided possessing a continuous longitudinally extending channel free from macromolecular material, said fiber consisting essentially of a copolymer of acrylonitrile and an olefinically unsaturated comonomer containing an optionally salified sulphonic acid group, and possessing micropores of average diameter less than about 100 A, between 40% and 80% of walls of the fiber being empty space. This fiber can be prepared by injecting a solution of a copolymer of acrylonitrile and an olefinically unsaturated comonomer containing an optionally salified sulphonic acid group in at least one polar organic solvent into a spinneret with an annular orifice, and immediately coagulating the inside and the outside of the nascent hollow fibre issuing from the spinneret with a coagulating fluid which is selected from:A. a less than saturated aqueous solution of an inorganic salt, said solution optionally containing up to 40% of a miscible polar organic solvent, andB.