Abstract Retention measurements with single salt solutions of CaCl 2, NaCl and Na 2SO 4 revealed that the rejection mechanism of commercial polymeric nanofiltration membranes investigated in this study may be divided into two categories: 1. Membranes for which Donnan exclusion seems to play an important role. 2. Membranes for which retention is determined by both Donnan exclusion and size effects. In category 1 both positively and negatively charged membranes were found. Ceramic γ-Al 2O 3 ultrafiltration membranes with a pore size of 3 nm showed a same type of salt retention behavior as the positively charged polymeric membranes. The extended Nernst–Planck equation in combination with the Donnan equilibrium has been used to model the flux-retention experiments for the salt solutions. The numerical calculations resulted in a good agreement with experimental data and acceptable values for the fixed charge densities have been determined. The effective membrane thicknesses calculated were higher than those observed by scanning electron microscopy.