Polyacrylonitrile copolymer membranes with filtration performance controlled by pH were prepared by phase inversion technique. The pH sensitive molecular sieve effect on transport of macromolecular solute was compared in both copolymer membranes having acrylic acid (AA) and methacrylic acid (MA) segments. The water permeation rate through the membranes was remarkably decreased by the surrounding pH change from acid to alkali condition. The molecular weight cut-off data obtained by the dextran permeation at pH 4, 6 and 10 indicated that pore size of the membrane is significantly reduced in the alkali condition. Without and with 0.1 M NaCl in the permeate solution, different molecular sieve effects were observed in the copolymer membrane with the AA segments, whereas the membrane with MA segments made no significant change. Evidence was presented that the carboxylic acid segmental conformation is associated with the pH sensitive molecular sieve effect of the copolymer membranes.