Sulfated chitosan (SCS), a typical amphoteric polyelectrolyte, was prepared by a homogeneous method. A novel kind of amphoteric composite nanofiltration (NF) membrane was prepared by coating the aqueous solution of sulfated chitosan onto a PAN UF membrane, subsequently crosslinked with glutaraldehyde. The effects of the membrane preparation techniques and operating conditions on the rejection performance of the composite membranes were studied. The results suggest that composite NF membranes with excellent rejection performance could be prepared while SCS concentration was 2.0 wt%, curing time was 1 h at 50 degrees C, glutaraldehyde concentration was 0.5 wt%, and crosslinking time was 1.5 h at 60 degrees C. The rejections of the resulting membrane of K2SO4, Na2SO4, and NaCl solutions (1000 mg/L) were 92.8, 92.1, and 50.3% at 25 degrees C and 0.40 MPa; the permeate fluxes were 11.3, 7.1, and 10.4 k g/m(2)center dot h, respectively. The rejection of this kind of membrane to the electrolyte solutions decreased in the order of K2SO4, Na2SO4, KCl, NaCl, MgSO4, and MgCl2. This suggests that the membrane's active layer acquires a negative surface charge distribution by the adsorption of anions from the electrolyte solution, and this charge distribution mainly determines the performance of the membrane.