Five NF membranes (NF 270, NF, Desal-5 DK, Desal KH and NF 20) were tested for their flux and selectivity, when solutions containing equimolar amounts of H2SO4 and CuSO4 were filtered at a set permeate flux (8 mu m/s). Significant differences were observed between the NF membranes of different manufacturers when acid retentions were compared. The Desal membranes (GE Osmonics) showed high H2SO4 retentions (70-90%) at low concentrations and 5-10% at a very low pH (0.35). For the other tested membranes, negative acid retentions were observed and acid retention was generally very low (-20 to 15%). The differences in the retention patterns between the membranes by the same manufacturer were marginal, which indicated that their surface layer structure was similar. An increase in the total ion concentration enhanced acid and salt permeation for the tested GE Osmonics membranes. At higher pH values the surface charge effects hindered ion transport. When very concentrated equimolar solutions of sulphuric acid and copper sulphate were filtered, the retained salt had no or very little effect on acid permeation. The results supported previous experiments that reported steric effects and diffusivity instead of charge effects as major factors determining selectivity when concentrated solutions were nanofiltered. (C) 2007 Elsevier B.V. All rights reserved.