The fluorescence probe method was applied to evaluate pH responsive filtration performance of polyacrylonitrile (PAN) membranes having molecular size exclusion effect. The following PAN terpolymers with carboxylic acid groups and fluorescent naphthalene moieties were synthesized by radical polymerization: poly(acrylonitrile-acrylic acid-2-vinylnaphthalene) and poly(acrylonitrile-methacrylic acid-2-vinylnaphthalene). Using these polymers, porous membranes were prepared by means of the phase inversion method and the molecular size exclusion effect was examined at various pHs under ultrafiltration operated at 2.5 kPa of applied pressure. In the same pH region, the fluorescence spectra of naphthalene moieties binding covalently with the membrane were measured as the excitation radiation was at 280 mu for the chromophore. The fluorescence intensity was decreased when the surrounding pH was changed from acid to alkali. In addition, red shift of the fluorescence maximum for the naphthalene probe moieties was observed in alkali condition. The data indicated that the nature of the probe environment in the membrane changed from non-polar medium to polar medium as the surrounding pH increased. It was shown that the dissociation of acrylic acid and methacrylic acid segments was influenced by the surrounding pH and the polymer membrane having such a segment showed pH responsive filtration. The evidence was supported by characterization of the membranes at various pHs. (C) 2000 Elsevier Science B.V. All rights reserved.