A systematic study on the zeta potential of polyethersulfone membranes determined by streaming potential measurements was made to examine various salt effects. Chloride salts; KCl, NaCl, CaCl2 and MgCl2 and potassium salts; KCl, KNO3, KClO3, K2CO3, K2SO4 and K2Cr2O7 were used to investigate the effects of cations and anions on the zeta potential of membranes, respectively. The zeta potential was obtained in 0.001 M salt solutions at the range of pH=4.0-8.0. The effect of ionic strength on membrane zeta potential was also investigated in the solution of KC1 and Na2HPO4 with 0.001 M-0.1 M salt concentrations. At high pH values, anion adsorption on membrane surface had a more potent effect on zeta potential, while at low pH values, cation adsorption to membrane surface also possessed more potent effect on zeta potential. In general, zeta potential decreased with increasing ionic strength due to the compression of electrical double layer. However, preferential adsorption of ions on the membrane surface is another important parameter affecting zeta potential beside the ionic strength. The influence of ionic strength on the membrane zeta potential can be observed by performing FTIR-ATR analysis.