A permselective membrance suitable for obtaining fresh water by the desalination of sea water and salt water is provided herein, said permselective membrane being prepared by the reaction of an aromatic polycarboxylic acid component with a mixed diamine component comprising a diaminodiphenylsulfone compound of the formula (I), ##STR1## wherein R.sup.1 is hydrogen atom or a C.sub.1 -C.sub.12 hydrocarbon group, R.sup.2 s may be same or different, each representing a monovalent organic group, and n is an integer of 0 or 1 to 3, and an alicyclic diamine compound containing two secondary amino groups in the same or different ring(s).