This work presents a detailed study on the pH sensitivity of polyethersulfone (PES) hollow fiber membrane (HFM) modified by blending an amphiphilic terpolymer poly(styrene-acrylic acid-N-vinylpyrrolidone) (P(St-AA-NVP)), the HFM exhibits a pH-responsive behavior with very large water flux change as the pH value change, from nearly 38.64 to 0.64 ml/(m(2) h mmHg). Meanwhile, the influence of the occurred macro- and micro-phase separation on water contact angle, pH sensitivity, and anti-fouling property was investigated. X-ray photoelectron spectroscopy analysis revealed the enrichment of the hydrophilic segments of the terpolymer at the membrane surface. A rough surface resulted from the poor miscibility of the blend was observed, and it was proved that the phase separation would increase the pH sensitivity. Furthermore, the ultrafiltration experiments indicated that the phase separation had great effect on the pure water flux but little influence on the protein anti-fouling property after the blending of the terpolymer; and the modified membrane showed good protein anti-fouling property. (C) 2011 Elsevier B.V. All rights reserved.