A comprehensive fractionation technique was applied to a set of water samples obtained along a real drinking water treatment plant with ozonation and granular activated carbon (GAC) treatment to obtain detailed profiles of dissolved organic matter (DOM) and to evaluate the haloacetic acid (HAA) formation potentials of these DOM fractions. The results indicated that ozonation and GAC treatment showed limited ability to remove hydrophilic fractions (23%), while removal of hydrophobic fractions was 72%. The contribution of hydrophilic fractions to HAA formation increased from 30 to 61% along the treatment train because of better removal for hydrophobic fractions both in concentration and reactivity. Similar trends were also found for trihalomethanes.