Because of the diversity of streaming potential and zeta potential values reported in the literature, even for similar membranes and external conditions (hydrodynamics and solution chemistry), the influence of measuring cells and procedures in electrokinetic parameters has been studied. For this purpose, two different experimental systems (named EKA and NIS) with similar hydrodynamic parameters, but different electrodes and electrical devices, are used for the electrokinetic characterization of a commercial polysulfone membrane. Detailed descriptions of measurement procedures and the influence of external conditions (flow direction, electrode polarization, etc.) are made since they could contribute to the variability of experimental results. On the other hand, an electrokinetic characterization of the polysulfone membrane in contact with NaCl solutions at different concentrations and pH was carried out by using the EKA system. An heterogeneous specific adsorption of anions on the membrane surface (Freundlich isotherm) was assumed in order to determine parameters such as the membrane surface charge density, the Gibb's free energy and the number of adsorption sites per unit of membrane area. However, at low salt concentration (similar to 10(-3) M) the membrane proper charge is mainly due to dissociation of some kind of acid groups on the membrane surface (isoelectric point: 3.1 +/-0.1), and the site dissociation model has been used to obtain the membrane acid equilibrium constant and the surface density of acid sites.