Drinking water resources contaminated by wastewater discharge could have great risk of trihalomethane (THM) formation upon chlorine disinfection during treatment. In the present study, THM formation potentials (THMFP) of the organic pollutants in the raw sewage and treated secondary effluent collected from f ull-scale wastewater treatment plants were characterised. The organic matter remaining in the secondary effluent had higher unit carbon THMFP in comparison with that in the raw sewage. However, owing to removal of organic pollutants in the treatment works, the overall THMFP of the wastewater was reduced significantly throughout the biological treatment process. Using XAD-8 resin, organic materials in the wastewater samples were separated into hydrophobic and hydrophilic fractions. Hydrophobic organic, which possesses higher THMFP than hydrophilic organic, was the predominant THM precursor in wastewater discharge. Ozonation could be used to alter the properties of organic matter, particularly the hydrophobic fraction, and reduce its THMFP.