A novel chemical modification process has been proposed to prepare polybenzimidazole (PBI) nanofiltration membrane with desirable pore sizes and pore-size distribution. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analysis have verified the proposed chemical modification reaction between the PBI membrane and p-xylylene dichloride. Depending on the post-treatment process in the p-xylylene dichloride/heptane solution, one can finely control the pure water permeability of the modified membrane. The modified PBI membranes have a decreased effective mean pore size, a narrowed pore size distribution, and demonstrate superior ion rejection performance for liquid separation, especially for the fractionation of multivalent cations and anions from monovalent ions. It is also found that the charge characteristics of PBI membrane may vary with the solution pH, which is reflected by the rejection of NaCl under different pH. Particularly, this modified PBI NF membrane can be employed for the separation and concentration of cephalexin with impressive high rejection rates under a wide range of pH. (c) 2006 Elsevier Ltd. All rights reserved.