The ion retention and flux of nanofiltration (NF) membranes are to a large extent determined by the membrane surface charge. This surface charge is in turn strongly influenced by adsorption of ions from the solution onto the membrane material. A 1-pK adsorption model with a Basic Stern electrostatic double layer model is used to describe ion adsorption, and the sensitivity of this model for various parameters is discussed. From a non-linear regression analysis of literature data [1,2] regarding the surface charge and the zeta-potential, adsorption parameters for the 1 pK model are obtained for sodium chloride on γ-alumina. The 1-pK Basic Stern model could predict the surface charge well, except for the highest concentration of 1000 mol/m3. Reasonable agreement is found between the measured zeta-potentials and the model predictions.