Permeation experiments of KCl, NaCl, LiCl, MgCl 2 , MgSO 4 and K 2 SO 4 solutions were carried out using two commercial nanofiltration membranes, which were NF45 (Dow Chemical Corporation) and SU200 (Toray Corporation) membranes. The effects of the type and the concentration of electrolytes as well as the pH value of the electrolyte solution on the separation performance (such as rejections) were investigated. The experimental results showed that NF45 and SU200 membranes possess similar separation potential to the inorganic electrolytes of 1-1 type (LiCl, NaCl and KCl) and much different ones to the other electrolytes containing bivalent ions (MgCl 2 , K 2 SO 4 and MgSO 4) with different concentrations. The rejections to inorganic electrolytes by the two NF membranes declined with the growth of the electrolyte concentrations and approached some certain values when on much high concentrations (more than 400 mol·m –3), which implied that the size of electrolyte-ions could not be ignored by comparison to the pore radii of NF membranes. The two NF membranes behave almost the same rejections to the electrolytes such as LiCl under various pH values of the feed solution from pH=4 to 10 and behaved the similar minimum values near the isoelectric point of pH=6.5.