The apparent surface charge densities of grafted poly(acrylonitrile) (PAN) membranes (determined by measurement of zeta potential) were compared with net charge densities (determined by potentiometric titration) in order to examine the effects of hydrophilic and hydrophobic graft chains at the pore surface. Membranes with hydrophilic graft chains showed much smaller net charge densities than membranes with hydrophobic graft chains. However, the apparent surface charge densities of the membranes with hydrophilic graft chains were much larger than those of the membranes with hydrophobic graft chains, This fact can be explained by the formation of ion pairs between charge groups and counterions, Dissociation behaviors for the two types of membranes, in which electrostatic interactions of the charge groups play a significant role were distinctly different. These results confirm the occurrence of ion-pairing effects between the charge groups and the counterions for hydrophobic graft chains in which dissociation of some of the charge groups is suppressed.