The Gouy-Chapman-Stern theory is extended, and analytic expressions are obtained for the ion binding constant (K) and the density of binding sites (N). Parameters of ion binding to dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) membranes are determined simultaneously from the extrema of the vesicle zeta (or surface) potential vs ion concentration curves without arbitrary suppositions concerning the ion-to-lipid binding ratio, using the values of the thickness of the shearing layer delta = 0 and 0.2 nm.