An asymmetric membrane formed from a sulphonated polyarylethersulphone has a salt rejection of at least 99%. Alternatively, a membrane has a good salt rejection and an acceptable water flux such that the ratio ##EQU1## has the value of at least 0.7. The membranes can be obtained using barium salts of the sulphonated polyarylethersulphone, with the proportion of barium being controlled to be at least 85%, preferably at least 86%, and not more than 91%, preferably not more than 90% of the barium required to react with the sulphonic acid groups on the sulphonated polymer.