Thin film composite (TFC) membranes with sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) as the top layer coated on ultrafiltration (UF) membranes were prepared under different preparation conditions including the polymer concentration, solvent used in the coating solution, drying temperature, and the polymer molecular weight. Membrane performances for nanofiltration that separated electrolytes such as MgSO4 and NaCl from aqueous solution were investigated using the composite membranes thus prepared. The concentration of SPPO both in H+-form and in Na+-form showed different tendencies for MgSO4 separation while it did not affect the separation of NaCl. Membrane permeation rate increased on increasing the molecular weight of the glycol ethers used as the solvent without sacrificing separation. TFC membranes prepared from high molecular weight SPPO exhibited better performances as compared to those prepared from lower molecular weight SPPO.