The surface charge densities acquired by intrinsically neutral and charged microporous membranes when bathed by diluted electrolytic solutions are studied. Concretely we deal here with (\mathrm{LiCl}) solutions and two nylon neutral microporous membranes and two very similar but charged membranes whose surface charges were previously obtained, within the frame of the so called microcapillary pore model. The dependence of the surface charge densities and the zeta potentials on the bulk concentration is studied by assuming that an adsorption process of anions is the cause of these charges. A general graphical method is proposed to distinguish between Langmuir and Freundlich adsorption. If this method is applied to the four nylon membranes, the underlying adsorption mechanism seems to be of the simple Langmuir type without heterogeneous adsorption. Thus, the free energy of specific adsorption, the maximal number of sites accessible to anions, and the proper surface charge density are evaluated. The first two parameters are very close for neutral and charged membranes, while the calculated proper charge is positive for the charged filters, as claimed by the manufacturers, but relatively small, and zero for the neutral ones. @ 1993 Academic Press, Inc.