The molecular weight cut-off (MWCO) is an important parameter used in the selection of membranes for specific filtration tasks. The MWCO is also used as a parameter in various membrane models. In this study the MWCO of an NTR 7450 nanofiltration membrane was investigated by filtration experiment and calculation of the rejection of neutral molecules as a function of solute molecular weight. The results were compared to a reflection coefficient model and it was shown that increasing the solute concentration decreased the MWCO value, indicating that MWCO was not a constant parameter but was dependent on concentration. The dipole moment of a solute molecule was also found to have an effect on the membrane's MWCO. The results showed that molecules with a large dipole moment caused rejection to increase with an increase in solute concentration. (C) 2014 Elsevier B.V. All rights reserved.