In this study high performance size exclusion chromatography (HPSEC) was used to compare an ultrafiltration (UF) membrane and alum coagulation for their capacity to remove different fractions of natural organic matter (NOM) from water. At the same time, the removal of disinfection by-product (DBP) precursors, as measured by trihalomethane formation potential (THMFP) and haloacetic acid formation potential (HAAFP), was also detected. The results show that the UF membrane mainly removed the aliphatic biopolymer fraction, while alum coagulation mainly removed the humic substances fraction. The results of DBP precursor analysis show that more THMFP was removed by the UF membrane than HAAFP, while the reverse was true for alum coagulation. It is conjectured that the aliphatic biopolymer fraction is the major precursor for trihalomethanes (THMs), while the humic substances fraction is the major precursor for haloacetic acids (HAAs).