Nanofiltration (NF) is receiving increased attention for drinking water applications due to the high capabilities of the process for removing NOM and the related disinfection by-products. Further understanding of the factors that influence NF membrane performance is necessary to optimize the process. The objectives of this research were to examine the effects of NOM composition on the performance of a nanofiltration membrane. The composition of NOM was manipulated by fractionation into hydrophilic and hydrophobic components and pretreatment with powdered activated carbon (PAC). Membrane performance was evaluated according to the rejection of NOM from the feed water and the time dependent decline in permeate water flux. NF experiments were conducted using a bench scale system arranged in batch, recycle mode. Operationally defined hydrophobic and hydrophilic NOM was obtained by fractionation of the NOM using XAD-8 resin. In the PAC pretreatment, a fixed dosage of PAC and varied contact times were used. The NOM solutions used as NF feed waters were diluted to the same TOC concentration in order to determine the effects of NOM composition on the NF performance. The hydrophobic NOM was more highly rejected by the NF membranes than the hydrophilic NOM. Results indicated that the rejection of the NOM fractions was highly influenced by the NOM hydrophobicity, as well as the apparent molecular weight distribution (AMWD) of the NOM. Increased PAC contact time in pretreatment resulted in a decreased rejection of the NOM remaining in solution. Differences in the rejection of NOM from the PAC pretreated solutions were primarily due to differences in the AMWDs of the NOM. The hydrophobic NOM caused substantial permeate flux decline; the hydrophilic NOM caused no permeate flux decline; and unfractionated NOM caused the more extensive flux decline than either of the two NOM fractions. Increased PAC contact time in pretreatment resulted in greater permeate flux decline caused by the NOM remaining in solution. Results suggested that the high molecular weight NOM (i.e., >30,000) was most responsible for permeate flux decline.