The apparent and effective molecular weight cut-off (MWCO) values of negatively charged nanofiltration (NF) membranes were determined using the fractional rejection of solutes of interest, including neutral polyethylene glycols and natural organic matter (NOM) containing ionizable functional groups. Varying the hydrodynamic operating conditions, denoted by the J/k ratio (where J represents the permeate water flux/convectional transport and k represents the mass transfer coefficient/back diffusional transport), produced changes in both the nominal and effective MWCO. Consequently, a higher J/k ratio resulted in reduced solute rejection and a higher MWCO. It was confirmed that the nominal MWCO provided by the membrane manufacturers might not be capable of predicting performance with respect to the different characteristics of various solutes (i.e., contaminants) since the predicted nominal MWCO values were nearly twice as high as those measured in this investigation. The effective MWCO values of the NF membranes with respect to NOM were slightly higher than the corresponding nominal MWCO when the NOM was relatively hydrophilic. (C) 2014 Elsevier B.V. All rights reserved.