In our recent study, pH-sensitive polyethersulfone (PES) hollow fiber membranes were prepared by blending poly (acrylonitrile-co-acrylic acid) (PANAA), and the electroviscous effect had great effect on the water flux change. While the question remains: is the water flux change caused by the electroviscous effect for all the membranes with different pore sizes? Herein, pH-sensitive hollow fiber membranes with different pore sizes were prepared. The pore size and the theoretic water flux were calculated through the ultrafiltration of polyethylene glycol (PEG) solution. Comparing the calculated fluxes and the experimental ones, we found that the water flux change was mainly caused by the pore size change at the pH value larger than pKa, while that was caused by both the pore size change and the electroviscous effect when pH value was smaller than the pKa, and the pore size change was caused by the ionization of the -COOH in the copolymer. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 2320-2329, 2012