The pore size distribution of a polysulfone membrane has been obtained from computerised image analysis of scanning force microscopy pictures. The streaming potential on the active layer of this membrane has been measured when it is in contact with aqueous NaCl solutions of different concentrations. Finally, NaCl retention has been taken into account to allow the translation of measured electric potential to the actual streaming potentials through the membrane. It has been shown that it is possible if the diffusion potentials appearing through both the concentration polarisation and the membrane itself are taken into account. By a fitting procedure, the effective transport numbers are obtained, which prove to be adequate not only for high concentrations, when there is no significative electrical double layer overlapping, but also for all the concentrations range used leading to the same streaming potential than measured on the membrane. The so-obtained effective cation transport numbers are not equal to the free solution true transport numbers that are defined when no pressure is applied. They are very similar for the cation and the anion, what should be caused by the applied pressure that should make their mobilities almost equal. Only a little contribution from the diffusion potentials, caused by retention without a relevant separation of the ions, should remain.