The surface electrochemistry of several microporous membranes is studied. Concretely, we deal here with six types of track-etched microporous filters from Cyclopore whose surface charges are obtained, within the framework of the so-called microcapillary pore model, from typical streaming potential experiments when bathed in diluted LiCl solutions. 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. The adsorption isotherm data are treated according to two adsorption mechanisms, a heterogeneous Freundlich adsorption followed by a homogeneous Langmuir adsorption for higher bulk concentrations. Both stages allow us to obtain important parameters concerning the membrane material : the molar free energy of specific adsorption, the maximum number of sites accessible to anions, and the actual surface charge density at the solid surface (or proper surface charge density). All these parameters are quite similar for all the membranes analyzed, showing significative differences depending on the adsorption step; in particular, fewer adsorption sites and higher adsorption free energies appear for high concentrations while the proper charge is positive but low and acts only for very low bulk concentrations. This two-step adsorption model gives a very approximate prediction of actual zeta potentials as well.