The effect of bromide ion, organic carbon concentration (natural aquatic humic substances), pH, and solar irradiation on the formation of bromoform in ozonated groundwater has been studied. The studies were conducted on four unique samples of groundwater taken from different regions of the Biscayne Aquifer in southern Florida. All other conditions being equal, increases in bromide ion concentrations resulted in increases in CHBrg formation. In three of the four samples, CHBr3formation decreased as the pH level increased from 5 to 9. The fourth sample exhibited an opposite trend whereby the CHBr3concentration increased with increasing pH. Bromoform concentration increased with increased O3concentration over an ozone dosage range of 3.4 to 6.7 mg/L.