Solutions of sulphonated polyaryletherketones in a solvent mixture of at least three liquids or low melting solids which are non-solvents or poor solvents for the polymer, can be used to produce asymmetric semi-permeable membranes. The polymer may be a polymer of(Ph.sup.1 --O--Ph.sup.2 --O--Ph.sup.1 --CO)and optionally(Ph.sup.1 --O--Ph.sup.1 --CO)and(Ph.sup.1 --O--Ph.sup.1 --O--Ph.sup.1 --CO)wherein Ph.sup.1 and Ph.sup.2 are respectively unsulphonated and sulphonated phenylene residues. The components of the solvent mixture have specified solubility parameters. A suitable solvent mixture is one formed from water, 1,4-dioxane and acetonitrile.