The presence of ammonia nitrogen (NH3-N) in the effluent strongly affected the formation of disinfection by-products (DBPs) during its chlorination. The effect of chlorine (as mg/L Cl-2) to NH3-N (as mg/L N) mass ratios (Cl-2/N) and the chemical fractions of dissolved organic matter (DOM) in the effluent on the DBPs formation was investigated. Results indicated that the formation of DBPs increased with increasing Cl-2/N. The concentration and speciation of DBPs varied among different DOM fractions at different zones of chlorination breakpoint curves. The formation rate of total haloacetic acids (THAA) and total trihalomethanes (TTHM) was promoted after the chlorination breakpoint, whereas the reaction of monochloramine with HOCl to dichloramine may cause a decrease in the DBPs formation potential thereafter. Organic acids were found to be the dominant precursors of DBPs with or without the presence of NH3-N, which indicated that the C=C, C=O and C-O structures contributed to the formation of DBPs significantly. In addition, the incorporation of bromine in THMs of the HiA fraction increased with the increasing of Cl-2/N mass ratios before the chlorination breakpoint, but decreased sharply after the breakpoint. Delta A(280) (absorbance at 280 nm), defined as A(280,initial)-A(280.final), was proved to be linearly related to the TTHM and THAA of wastewater without containing Br- during chlorination or chloramination. (C) 2011 Elsevier B.V. All rights reserved.