A polymer solution contains a sulphonated polyarylethersulphone in a solvent mixture containing at least three components, where the solvent mixture has a delta-H in the range from 3 to 8.5; a delta-P in the range from 4 to 8 and a delta-D in the range 7.2 to 9.5 and each component of the solvent mixture is a liquid or low melting solid which is a non-solvent or poor solvent for the sulphonated polyarylethersulphone, and at least one of the components satisfies at least one of the conditions:a) said component has a delta-D of less than 8 when the value of delta-P is not more than 3; andb) said component has a delta-H of greater than 3 when the value of delta-P is at least 8.5; andc) said component has a delta-H of less than 8 when it contains at least one hydroxylic group; andd) said component is free of hydroxylic groups and has a delta-P which is greater than 3 and less than 8.5.The solution can be used for the production of membranes suitable for use in reverse osmosis applications.