Abstract: A process for preparing a composite membrane comprising acidic and/or basic groups comprising the following steps: (i) applying a curable composition to a support; and (ii) curing the composition to form a membrane; wherein the curable composition comprises: (a) 18 to 78 wt % crosslinking agent(s) having at least two acrylic groups; (b) 20 to 80 wt % curable compound(s) having one or more acidic or basic groups or groups which are convertible to acidic or basic groups; (c) 2 to 30 wt % solvent(s); and (d) 0 to 10 wt % photoinitiator(s). The membranes are particularly useful for water purification or for the generation of electricity from two streams differing in salt concentration.