The reverse osmosis performance of thin-film composite membranes prepared from sulfonated polyphenylene oxide polymer with different ion-exchange capacities was studied using various electrolyte solutes. The effect of the solvent used for the preparation of the polymer solution for surface coating was also studied. It was found that the preparation of thin-film composite membranes with high selectivities and high fluxes was possible by adjusting properly the ion-exchange capacity and the solvent. It was also found that the membrane performance was governed primarily by the ion-exchange reaction between the solute cation and the proton in SO3H and the Donnan equilibrium.