Many source waters in China are in high levels of natural organic matter (NOM) and bromide ion (Br-), and the control of the formation of regulated and non-regulated disinfection by-products (DBPs) is a great challenge. This study uses the natural water with substantial NOM and Br- from Weishan Lake to evaluate the formation and species transformation of DBPs in three disinfection scenarios of chlorination, chloramination, and chlorine-chloramine sequential treatment (Cl-2-NH2Cl process), and the reaction pathways involved are also proposed. Compared with chlorination, the formation of trihalomethanes and haloacetic acids is reduced by 88% and 85% and by 68% and 65% in chloramination and Cl-2-NH2Cl process, respectively. During chloramination, the proportion of brominated-DBPs (Br-DBPs) is lower initially, and then increases afterwards due to the continuous reaction between NOM slow reactive sites and N-bromo amines. Additionally, the ratios of dihalogenated acetic acids to total organic halogen increase continuously from 4.5% to 6.3% with pre-chlorination and from 4.3% to 9.8% without pre-chlorination. The halonitriles (HANs) formation during chlorination is the most significant in the initial 30 min at the level of 6.5 nM, and then decreases to 2.5 nM after 72 h. During chloramination, however, the formation of HANs increases steadily with prolonged contact time. After 72-h contact time, the level of HANs in Cl-2-NH2Cl process is 3.9 and 2.5 times of that in chlorination and chloramination, respectively. In Cl-2-NH2Cl process, HOCl rapidly oxidizes Br- to HOBr, and these two active species react with the fast reactive sites within NOM to form Br-DBPs and the intermediates for nitrogenous DBPs (N-DBPs). The subsequent introduction of ammonia inhibits the hydrolysis and oxidation of the formed N-DBPs and contributes to the most significant formation of N-DBPs. (C) 2012 Elsevier B.V. All rights reserved.