Ion permeation experiments across a nanofiltration membrane were performed with NaCI at several pHs. A remarkable dependence of ion rejection on pH is observed. This phenomenon is considered to be caused by the change of surface charge, which results from the adsorption of ions onto the membrane pore wall. A model, which describes the diffusion of free ions within the membrane by the Nernst-Planck equation and adsorption of ions on the surface of the membrane by a Langmuir-type adsorption isotherm, is successfully applied to explain the experimental data.