In this work the effect of pH on membrane structure, its permeability and retention was studied. In addition, we studied whether the possible changes in the membrane properties due to the pH change are reversible. This is important for understanding the performance of nanofiltration membranes at different conditions and for the selection of cleaning processes. Moreover, the results facilitate the choice of membrane for specific applications. Several commercial NF membranes were studied at different pH values. Their retention and flux were explained by the charge and the hydrophilic characteristics of the membranes. The filtrations were made with uncharged sugar and salt solutions. The lower the membrane contact angle (i.e., a more hydrophilic membrane) the higher was the change in apparent zeta potential when pH was increased from 4 to 7. As a result, the retention of ions with more hydrophilic membranes changed more than hydrophobic ones when the pH was increased in the feed solution. However, some membranes retained ions well at high pH although their apparent zeta potential or hydrophilicity was relatively low. These membranes had charge inside the pores and it was not detected by streaming potential measurement along the surface or by measuring the contact angle of the surface. Thus, the apparent zeta potential of the exterior membrane surface did not sufficiently describe the ionic transport through the membrane. In addition, some membranes became significantly more open at high pH (i.e., flux increased). This was explained by the chemical nature of the polymer chains in the membrane skin layer, i.e., dissociating groups in the polymer made the surface more hydrophilic and looser when charges of the polymer chains started to repel each other at elevated pH. Generally, the retention of uncharged glucose decreased more at high pH than the salt retention. The changes in permeabilities and retentions were found to be mostly reversible in the pH range studied (very slowly in some cases, however). (c) 2006 Elsevier B.V. All rights reserved.