A mathematical model that expresses total trihalomethane (TTHM) concentration in terms of initial chlorine concentration, total organic carbon, bromide ion concentration, contact time, and pH is developed for Zai water treatment plant which supplies water to Jabal Amman. Laboratory studies of THMs formation upon chlorination and bromide ion addition at different points of Zai treatment plant were performed.Results showed an increase in THMs concentration upon increasing contact time and chlorine dose. TTHM in the effluent water from Zai treatment plant ranged from 22 ppb to 63 ppb; these values may be attributed to the low contact time and chlorine dose. Water samples taken from Jabal Amman gave TTHM in the range of 86 ppb to 181 ppb; these high values are due to long contact times, and rising temperatures.Predicted TTHM concentrations were compared to actual TTHM concentrations measured during the sampling program. Results showed that there is good agreement between measured and calculated TTHM concentrations, which means that the method presented in this book, can be used to obtain good estimates of TTHM concentrations throughout networks.