The use of ozone combined with ultraviolet radiation has been studied at the pilot-scale for removing trihalomethane (THM) precursors from potable water. The effects of variations in ozone dose rate, UV intensity and other parameters were first studied using a synthetic feedwater. The pilot plant was then operated for 8 months at the Sabine River Water Treatment Plant in Longview, TX. Kinetic analysis was performed on data collected during ozone photolysis experiments in the presence and absence of known hydroxyl radical scavengers. This analysis indicated that ozone photolysis in aqueous solution leads directly to the formation of hydrogen peroxide, which then produces hydroxyl radical by secondary reaction of peroxy anion (HO2)(-1) and subsequent species with ozone. The mechanistic results predict a maximum yield of 2/3 hydroxyl radical for each ozone molecule that is consumed by photolysis or subsequent reaction. The mechanistic results also predict a plateau in the hydroxyl radical yield as a function of the fraction of ozone photolyzed. The plateau in the hydroxyl radical yield in turn implies a plateau in treatment efficiency for organic compounds. A simple model was developed for ozone mass transfer with chemical and photochemical reaction. Using pilot plant data, a treatment cost projection for removing THMFP by photolytic ozonation was made. At 1-MGD capacity, projected treatment costs were 55 cents, 70 cents and 90 cents per 1000 gal. for 60%, 70% and 80% THMFP removal respectively, assuming an electrical energy cost of 10 cents per kilowatt-hour.