Nanofiltration (NF) membranes are increasingly becoming an option for drinking water treatment. Large organic compounds like polysaccharides and humics are clearly retained by this kind of membrane. The question whether nanofiltration offers an appropriate barrier for low molecular weight (LMW) organic compounds is still controversial. These compounds play an important role in terms of microbial regrowth potential. Previous studies (Escobar et al., 2000; Lee et al., 2005; Park et al., 2005) have already assessed this problem, focusing their research on either the characterization of the natural organic matter (NOM) or on the microbial regrowth potential. Only few data presenting simultaneous analysis of the fractions of NOM were currently available. In this study, the concentration of different fractions of organic matter and the assimilable organic carbon (AOC) fraction of different waters processed by nanofiltration as well as, for comparison, reverse osmosis were measured. Tests on flat sheet membranes using natural water with and without addition of low organic acids were performed. The goal was to investigate to what extent LMW organics and AOC are retained by NF. Includes 6 references, figures.