This study intends to monitor and evaluate the THMs concentration in the 6 different water districts in the city of Tehran as well as one district outside the city water district but inside the city limits. THMs are the main disinfectant by products (DBP) of chlorination, which are suspected to be human carcinogens. Formation of Trihalomethanes (THM) in drinking water depends mostly on raw water qualities such as pH, temperature, treatment process, residence time of water in the distribution system, as well as the bromine, chlorine and naturally occurring organic matter (NOM) concentrations. Because of the low cost of chlorine and convenience of application, chlorination is the most widely used disinfection process for drinking water….