PURPOSE: To improve the rate of water permeation of a permselective membrane and to enable the removal of materials having several thousands mol.wts. or below by using linear polysulfone wherein a part of the arom. groups in the repetitive unit contg. the arom. groups in the principal chain is subjected to acyl substitution.

CONSTITUTION: This permoselective membrane consists of the substantially linear polysulfone consisting of the repetitive unit expressed by the formula I (E1, E2, G1 and G2 are the arom. groups that may be the same or different, A denotes a valence bond or bivalent org. group). The above-mentioned arom. groups are acyl-substd. so as to have an acyl group within a 0.1W10 millimol for each 1g of a polymer.