For a given membrane and a given electrolyte, salt rejection depends on the feed concentration, the permeate flux and the pH value of the solution. The intrinsic separation factor increases with increasing permeate flux irrespective of the membrane''s surface charge. The strong dependency of the separation upon feed concentration is only found when the membrane is negatively charged. The dependency of intrinsic separation factor on the solution pH value varies greatly with the electrolyte types, and the separation dependency on the electrolyte valence types is finally determined by the charge of membrane.