A porous membrane is produced by gelation in a non-solvent medium of an aromatic ether ketone polymer dissolved in a strong acid which is substantially non-sulphonating to the polymer. The polyether ketone solution includes a polymer additive which is not substantially degraded by the strong acid and which is soluble both in the strong acid, the non-solvent and mixtures thereof. The gelation medium may, for example, be aqueous and the polymer additive may be polyvinylpyrrolidone. Membranes produced by the method of the invention have good overall performance and flux at any given molecular weight cut-off.