Charged ultrafiltration membranes of poly(acrylonitrile-co-styrenesulfonate) having an amphiphilic counter ion were prepared by ion exchange with quaternary ammonium salts such as stearyltrimethylammonium chloride. The ultrafiltration behavior of the charged membranes having amphiphilic groups was examined and compared with those of the corresponding sodium salt. From the uptake of a fluorescence probe, 8-anilino-1-naphthalenesulfonic acid sodium salt, by the membranes, it was proved that the hydrophobicity of the charged membrane is large in the amphiphilic quaternary ammonium salt compared with that for the sodium salt. The ultrafiltration rate of the charged membrane and the permeability of dextrans increased in the amphiphilic quaternary ammonium salt. Electrostatic sieve separation of dextran/dextran sulfate mixtures having similar molecular size was also investigated as a function of the content of styrenesulfonate group in the membrane. The separation of the mixtures was enhanced as the charge content of the membrane increased.