Abstract: A sulphonated aromatic polymer containing repeating units (Ar-Y) (where Y is SO.sub.2 and/or CO), where at least half of groups Ar contain a group SO.sub.3 M and M is l/m of a polyvalent metal cation in at least 80% of such groups SO.sub.3 M, and where m is the valency of the ion. The polyvalent cation compound is formed by heating the corresponding sulphonic acid or monovalent cation compound with a salt of a polyvalent metal. The polymer is useful as a membrane and the polyvalent metal salt addition and/or the heat treatment may be applied to polymer in membrane form.