The pH-sensitivity of two kinds of polyethersulfone (PES) hollow fiber membranes modified by two random copolymers, poly (acrylonitrile-co-acrylic acid) (P(AN-AA)) and poly(acrylonitrile-co-methyl acrylic acid) (P(AN-MAA)), was studied. The P(AN-AA) and P(AN-MAA) copolymers which had the same hydrophobic chains, comparable amounts of carboxyl groups but different hydrophilic chains, were synthesized via free radical polymerization and characterized by FT-IR, element analysis and H-1 NMR. The copolymers could be directly blended with polyethersulfone to prepare hollow fiber membranes by dry-wet spinning technique. Both of the two copolymer modified PES hollow fiber membranes showed evident pH sensitivity. The flux variation for the P(AN-AA) modified PES hollow fiber was larger than that for the P(AN-MAA) between the pH values 2.0 and 11.0, since the -CH3 affected the dissociation of the carboxyl, and further affected the pH sensitivity of the modified hollow fiber membranes. (C) 2011 Elsevier B.V. All rights reserved.