The partition of ions to zwitterionic surfaces is studied chromatographically and evaluated on the bases of two developed models, the partition and the ion-pair models, which are derived from the Poisson-Boltzmann theory. The zwitterionic surfaces are prepared by the adsorption of a zwitterionic surfactant (3-(N-dodecyl-N,N-dimethylammonio)propane-1-sulfonate) on octylsilanized silica gel. Some characteristic behaviors are observed : (1) the retention of a probe anion, I-, shows a maximum at a concentration of salts added in mobile phases and is affected by the nature of mobile phase anions, (2) cations do not affect the retention of the probe ion, and (3) the adsorption isotherm of a small and well-hydrated anion, e.g., Br-, bends upward and cannot be explained by the usual Langmuir-type equations. Although the equations derived on the basis of the constant dipolar layer thickness cannot explain some of results, better interpretations are possible by taking the salt-induced extension of the dipolar layer into account. In water, small and well-hydrated ions mainly interact with the zwitterionic surfaces by the partition mechanism, whereas large and poorly hydrated ions interact by the ion-pair mechanism.