A study on the performances of nanofiltration Desal-5 DK membranes is presented in the case of CaCl2-water mixtures. Extensive experimentation at room temperature is performed to characterize the membrane separation efficiency in a wide range of electrolyte composition (1-500 mol/m(3)) at various feed pH values (5-6.5). The applied pressure in the feed side is kept in the range from 3 to 30 bar. The effect of pH and concentration is investigated both on salt rejection and on total volume flux. The results are exhaustive and useful to draw general trends. Calcium rejection increases as the concentration increases and decreases as the feed pH increases. However, as the salt concentration increases, calcium rejection goes through a maximum value; at asymptotic conditions the maximum value is located at a concentration close to 100 mol/m(3) CaCl2. The effect occurs in the whole pressure range investigated and it is much more remarkable at the higher pH values. Volumetric membrane charge values corresponding to each operative conditions experimented are calculated through the "integral" version of the Dorman Steric Pore Model and Dielectric Exclusion. Membrane charge is obtained as an increasing function with pH, whereas it shows an unimodal behaviour in the concentration range investigated. At low concentrations the membrane charge is negative, it switches to positive values as the concentration increases and finally decreases till to negative values at higher concentrations. Amphoteric behaviour of polyamide Desal membranes is re-confirmed, although a remarkable dependence of the points of zero charge is obtained upon salt concentration. (c) 2006 Elsevier B.V All rights reserved.