In this study, a simplified method to prepare pH-sensitive polyethersulfone (PES) hollow fiber membranes was provided by blending with a copolymer of acrylonitrile and acrylic acid (PANAA). The copolymer PANAA was synthesized by free radical solution polymerization using N-methyl pyrrolidone (NMP) as the solvent. The PANAA copolymer solution can be directly blended with PES solution (using NMP as the solvent) to prepare hollow fibers by using a dry-wet spinning technique based on the liquid-liquid phase separation technique. The blended PES hollow fiber membranes showed evident pH-sensitivity and pH reversibility; and the pH-valve effect was observed at the pH between 4.5 and 11.0. We also proved that the electroviscous effects played the most important role on the pH-sensitivity and flux control for the blended membranes. (C) 2009 Elsevier B.V. All rights reserved.