Abstract: A semipermeable membrane having a surface comprised of a poly(arylacetylene) having at least a part of its benzene nuclei introduced with sulfonic acid groups. The poly(arylacetylene) is represented by the following structural formula (I): ##STR1## wherein X stands for Cl, Br or CH.sub.3, R stands for H or CH.sub.3 and n is a positive integer. The semipermeable membrane may be obtained by subjecting a membrane formed of the poly(arylacetylene) of the formula (I) to sulfonation conditions or by sulfonating the poly(arylacetylene) of the formula (I), and forming the sulfonated product into membrane.