Thin film composite membranes were prepared by coating a thin layer of sulfonated poly (1,4-dimethyl-2,6-phenylene oxide) polymer on the surface of a polysulfone ultrafiltration membrane. The membrane performance was studied when the sulfonate was either in the hydrogen form or loaded fully with an alkali metal cation. It was found that the permeation rate was greater than the pure water permeation rate, when the sulfonate was in the hydrogen form and the feed solution contained an alkali metal chloride solute. The permeation rate increased with an increase in the ionic radius of the alkali metal cation. With respect to membranes in which the sulfonate was loaded with an alkali metal cation, the pure water permeation rate was found to increase with a decrease in the ionic radius of the alkali metal cation. The permeation rate was the same as the pure water permeation rate when the membranes of the latter form were tested for reverse osmosis of aqueous solutions involving either sodium chloride or an organic solute.