Thermodynamic reversibility of ion binding at a membrane/aqueous interface has been demonstrated using-variable period X-ray standing waves generated above a gold mirror. The interface consists of-a negatively charged and polymerized phospholipid monolayer bathed in a dilute zinc chloride solution. Zinc ion distribution in the diffuse double layer was monitored as the pH of the aqueous medium was titrated from a value of 5.8 to 2.0 and back again to pH 5.8. The ion distribution was modeled by an exponentially decaying function. As Hf concentration increased upon changing pH from 5.8 to 2.0, the amount of zinc in the diffuse double layer (integrated as a two-dimensional projection along the surface normal) decreased from 1.4 x 10(13) to 1.0 x 10(12) ions/cm(2) and the concentration of bound zinc at the surface dropped by a factor of 6.4 from an initial value of 122 mM to a low of ca. 19 mM. Recovery of the pH 5.8 distribution profile was essentially complete upon back-titration from pH 2.0. These results demonstrate clearly that within the experimental error of the measurement ion distribution at the charged membrane interface is reversible upon adjusting the pH of the electrolyte solution which Corresponds to altering the degree of protonation and, thus, the net charge on the exposed phosphate headgroups.