A bench scale laboratory ozonator was constructed and determined to produce 27 mg O3/hr. A contacting system and experimental procedure was developed to approximate a mass balance of 0, 1, 2, 4, 8 and 16 ppm were applied to raw and magnesium treated water from Lake Washington which is the potable water source for Melbourne, Florida. The effect of ozone on trihalomethane precursors was studied by comparing THM formation at 1, 24, 48, 72, and 96 hours following chlorination. The concentrations of each specie, chloroform, bromodichloromethane, dibromochlormethane and bromoform are reported. The highest 96 hour TTHM concentrations reported were 2773 ppb for the raw and 1205 ppb for the treated water. Chloroform was found to be the major specie of the THM's, averaging 80%. Ozone was found to reduce color of the raw and treated Lake Washington water. Ozone was shown to neither increase nor decrease TTHM concentrations with utilized ozone dosed up to 10.6 ppm. Increasing utilization of ozone was found to increase CHClBr2 concentrations in the raw and coagulated water. Regression equations at the 0.01 level of significance were developed for CHClBr2 formation and utilized ozone, color reduction and utilized ozone, TTHM formation and reaction time, and finally ozone production and time.