Abstract: Dry semipermeable membranes of polyacrylonitrile or copolymers containing more than 65 wt. % of acrylonitrile are prepared by dissolving the polymer in a solvent in a solids concentration of 15-30% by weight; uniformly coating the solution onto a surface in a desired shape under an atmosphere whose relative humidity is 60-85% and dipping said coated surface into a non-solvent medium so as to remove the solvent which forms the membrane; heat-treating the membrane under moist conditions at 60.degree.-90.degree. C; and drying the membrane at a temperature less than the temperature of the heat treatment under conditions such that the coefficient of contraction is less than 1%, whereby a dried semi-permeable membrane having an open-celled structure with a pore size of a diameter less than 0.5 .mu. and a porosity of 0.40-0.7 is obtained.