Retention measurements with salt solutions of NaCl, Na2SO4, MgCl2, MgSO4 and LaCl3 were carried out for four commercial nanofiltration membranes. The retention of ionic components was analyzed by the Donnan-steric partitioning pare model (DSPM), that describes the solute transport through a membrane using the extended Nernst-Planck equation. The analysis made it possible to evaluate the membrane charge density, which showed that the charge density is not constant but depends very much on the salt and its concentration; this phenomenon was found for all membranes and is attributed to ion adsorption on the membrane material. For magnesium and lanthanum salts this could lead to a change in the sign of the membrane charge from a negative to a positive value. Theoretical calculations strengthened this finding.