Most conventional nanofiltration (NF) hollow-fiber membranes are fabricated by means or multiple steps of posttreatments. Because of the unique amphoteric characteristics of the imidazole group in polybenzimidazole (PBI), a novel PBI NF hollow-fiber membrane with appropriate charge characteristics has been fabricated by a one-step phase-inversion process without posttreatment. The resultant mechanically stable PBI membranes can withstand pressures up to 30 bars. It is found that the mean effective pore size decreases, whereas the pure water permeability increases with an increase in elongational draw ratio. The novel PBI NF hollow-fiber membrane exhibits higher rejection to divalent cations, lower rejection to divalent anions, and the lowest rejection to monovalent ions at pH 7.0. Thus the divalent and monovalent ions of both NaCl/MgCl2 and NaCl/Na2SO4 binary salt solutions can be effectively fractionated by the PBI NF membrane arising from ion competition. The surface charge characteristics of the PBI NF membrane are strongly dependent on pH. Therefore, by adjusting the pH of cephalexin aqueous solution to modify the ionization states of these dipolar cephalexin molecules, PBI NF membranes show high separation effectiveness for the recovery of this antibiotic from solution over a wide range of pH values. (c) 2005 American Institute of Chemical Engineers