Dissolved organic matter in the secondary effluent from the W wastewater treatment plant (Harbin) 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 distribution, trihalomethane formation potential (THMFP), UV-Vis and FTIR spectra of organic fractions in the secondary effluent were examined. XAD fractionation results showed that HPO-A and HPI dominated in the secondary effluent, collectively accounting for about 70.8%. HPO-A was the main precursors for THMFP following chlorination. HPO-A had the highest specific THMFP (STHMFP) and averaged 232.9 microg/mg. The STHMFP of HPO-N, TPI-A, TPI-N and HPI ranged from 100 microg/mg to 125 microg/mg. Chlorination resulted in increased UV-Vis absorbance for TPI-N and decreased UV-Vis absorbance for HPO-A, HPO-N, TPI-A and HPI. After chlorination, O--H, C=C and aromatic ring were found to decrease while C--O was observed to increase for HPO-A, HPO-N, TPI-A and TPI-N. In addition,C=O and C--Cl were found to occur as by-products from the chlorination reaction.