In the preparation of ultrafiltration membranes, the effect of the addition of ZnCl2 in a casting solution, polysulfone (PSf)/N-methylpyrrolidone (NMP) solution, was investigated on the membrane performance. PSf is rin uncharged polymer, but PSf/NMP with ZnCl2 showed an electroviscous behavior. The addition of 3 wt% ZnCl2 to PSf induced a red shift of 3 cm(-1) in the 1152.2 cm(-1) peak which was assigned to a symmetric vibrational stretching because of ZnCl2 interacted with PSf chain. The addition of ZnCl2 increased the reduced viscosity and decreased the critical concentration, resulting from the association between extended chains. The PSf/NMP solution with ZnCl2 required less water for the precipitation point than saltless solution did. It was observed that the addition of ZnCl2 increased the rejection rate and lowered the molecular weight cut-off (MWCO) of PSf membrane.