Here we will analyze the zeta potential versus pH characteristics of an ultrafiltration polysulfone membrane, that will be called here PRI-PSUF This membrane was prepared by phase inversion from casting solutions of polysulfone. The electric potential per unit pressure both through and on the membrane have been evaluated for a constant Cl- concentration with a gradual substitution of protons by sodium cations. It has been seen that the difference in potentials on and through the membrane is almost constant when pH is over 5 when the concentration of protons is very low. This difference can be attributed to diffusion potentials appearing through the membrane due to retention. While, when the fraction of protons is high the potentials through and on the membrane are substantially equal and very low. In order to obtain accurate zeta potentials and the isoelectric point, the streaming potential should be measured on the membrane, when no differences in the surface and inside materials should be expected. These measurements should be conducted at relatively high concentrations of a salt with very similarly mobile anion and cation and then extrapolated to zero protons percentage. All these conditions would warrant an easy calculation of zeta potentials by the simple Smoluchowski equation.