The role of bromide ion in the trihalomethane (THM) formation and structure of dissolved organic matter (DOM) during chlorination of the secondary effluent taken from the Wenchang Wastewater Treatment Plant (Harbin, China) was investigated. DOM was fractionated using XAD resins into five fractions: hydrophobic acid (HPO-A), hydrophobic neutral (HPO-N), transphilic acid (TPI-A), transphilic neutral (TPI-N) and hydrophilic fraction (HPI). The patterns of individual THM species with increased bromide concentrations were similar for all DOM fractions. The THM speciation as well as halogen fraction for these five fractions followed similar trends with the Br-/C12 ratio. Chlorination resulted in decreased ultraviolet (UV) absorbance across wavelengths from 250 to 280 nm for DOM fractions whether bromide ions existed or not, and bromide addition led to lower differential UV absorbance values. Fouri er-trans form infrared (FT-11R) results indicated that chlorination, whether bromide ions existed or not, resulted in the near elimination of aromatic C-H and amide peaks, increased C-0 absorption intensity and occurrence of C=O and C-Cl peaks for HPO-A. HPO-N, TPI-A and TPI-N. Furthermore, bromide addition in chlorination led to the occurrence of C-Br peak for all four fractions. (c) 2007 Elsevier 13N. All rights reserved.