pH-sensitive poly(acrylic acid)-grafted membranes were prepared by an ultraviolet irradiation technique in air at room temperature. Peroxides produced on the membrane surface was quantified by 1,1-diphenyl-2-picrylhydrazyl (DPPH), which indicated that there existed an effective irradiation time for graft polymerization. Through X-ray photoelectron spectroscopy (XPS) measurements, the effective thickness of poly(acrylic acid) grafted onto the surface of a polysulfone membrane was determined to be around 80-100 nm. pH-dependent permeation behavior was noted from the permeation of riboflavin as a model solute. A distinctive flux decline of riboflavin appeared in the range of pH 4-5. Therefore, the flux of the intelligent membrane was controlled by changes in the pH of the external solution, attributed to the changes in the conformation of the grafted chains of poly(acrylic acid), because of the electrostatic force among the dissociated carboxyl groups.