Reverse osmosis membranes

In the production of a porous membrane for use in the separation of solutes from solvents by reverse osmosis which comprises:

• dissolving a film forming cellulosic ester or ether and a water soluble pore producing compound in a volatile organic solvent to form a casting solution,
• casting said solution on a casting surface in a casting atmosphere to form a membrane of substantially uniform thickness,
• evaporating a portion of said organic solvent from said cast membrane and
• immersing said cast membrane in cold water to effect gelation thereof and removal of said pore producing compound therefrom,
• the improvement in which said membrane is cast from a solution containing 10-20% film forming compound without forced air circulation while maintaining a casting solution temperature of 0.degree. C or greater and a temperature differential of from 8.degree. C to 27.degree. C between said atmosphere and the temperature of said casting solution on said casting surface, the temperature of said atmosphere being the higher, so as to adjust the evaporation rate of the solvent from said cast membrane towards an optimum for the membrane casting solution, thereby to produce a membrane capable of allowing a higher product rate during the reverse osmotic separation at essentially the same separation factor.