Two microporous polyethersulphone filters are studied. The pore size distributions are obtained by an air-liquid displacement method. These distributions are narrow Gaussians with mean pore sizes over the nominal ones. The membranes are bathed by several diluted aqueous solutions of NaCl at 298 K. The equations of transport through the Gaussian distributed capillary pores of these membranes (Nernst-Planck, Navier-Stokes, Poisson, and charge and mass conservation laws) are numerically solved with the adequate limit conditions and used to obtain the electrokinetic coefficients for an assumed pore charge. The streaming potential is measured allowing to obtain the adsorbed surface charge density as a function of concentration (adsorption isotherm). The so obtained results are seen to deviate only slightly from those that should be obtained if all the pores were assumed equal to the most probable one (homoporous membrane). A Langmuir chloride adsorption mechanism fits well to the complete adsorption isotherm obtained; while a Freundlich or heterogeneous adsorption pattern could be assumed for low concentrations. Moreover, by an adequate correction of both the Langmuir and Freundlich equations to take into account the complex structure of the electrical double layer, the proper charges of the membranes are evaluated. These proper charges seem to be negligible in our case. Whereas, the average adsorption free energy and the maximum number of accessible sites are evaluated being relatively small, according to the hydrophillic character assumed for the membranes.