Dissolved natural organic matter (DOM) in a filtered river water was isolated and fractionated into six different fractions. Trihalomethanes (THMs) and haloacetic acids (HAAs) formed from these isolated DOM fractions during chlorination and chloramination were determined. Results show that the hydrophobic acid, hydrophilic acid, hydrophilic base and hydrophobic neutral are major precursors of THMs and HAAs. There exist good correlations between the values of specific ultraviolet absorbance at 254 nm of the individual DOM fractions and their disinfection by-products formation potential, indicating that aromatic moieties are responsible for disinfection by-products formation for both hydrophobic and hydrophilic DOM fractions. Chloramination of the DOM fractions yields much less THMs and HAAs than chlorination. For the dominant DOM fraction (i.e. hydrophobic acid) in the water, the yields of THMs and HAAs increase more significantly in chlorination than those in chloramination with the increase of disinfectant dosage, contact time and dissolved organic carbon content. (C) 2008 Elsevier B.V. All rights reserved.