The: main objective of this work is the determination of the effective surface charge of commercial nanofiltration (NF) membranes through streaming potential measurements in order to prove the existence of a relationship between the membrane surface charge, C-M, and the feed solution concentration, C-f, of the type of a Freundlich isotherm: In [C-M]= a + b In C-f. This relationship was an important finding of a model developed previously, which describes the NF mass transfer mechanisms and predicts the NF performance of single salt solutions. Two loose NF membranes, Desal G-10 and Desal G-20, are characterised as for the hydraulic permeability, molecular weight cut-off (MWCO), and ion exchange capacity (IEC). Permeation experiments of NaCl, Na2SO4, MgCl2 and MgSO4 solutions, in the range of concentrations 0.0002-0.02 N, are carried out in a DOW pilot unit with a plate-and-frame module, the permeate fluxes and the salt rejections being measured for pressures between 2.5 and 12.5 bar, at constant flow rate, pH, and temperature. The streaming potential developed in the salt solutions flowing parallel to the membranes is measured by an electro-kinetic-analyser (EKA, Anton Paar). This equipment automatically calculates the zeta potential and corrects it for the membrane surface conductance. The membrane surface charge is determined by using the Gouy-Chapman equation. The membrane Desal G-20, the one bearing a higher surface charge, appears to display a logarithmic relationship for NaCl: In [C-M] (mol/m(3)) = 3.57 + 0.475 In C-f (mol/m3), though in a smaller concentration range than intended to be checked, due to the scattering of zeta potential data above 0.005 N. Furthermore, the surface charge of Desal G-20 is likely to be partially due to adsorption of ions since its IEC is lower than the surface charge determined by electrokinetic measurements at some experimental conditions.