Control of disinfection by-products (DBPs) in drinking water is becoming increasingly critical as regulatory requirements are calling for higher levels of disinfection while at the same time mandating lower maximum contaminant levels of DBPs. Many utilities have switched to alternative disinfectants such as ozone and chlorine dioxide (ClO2) to minimize chlorinated DBPs. Contra Costa Water District undertook a full-scale demonstration study using ClO2 and ozone to evaluate the potential benefits of combining these two powerful oxidants. The principal research objective, stated broadly, was to evaluate the benefits of using ClO2 in combination with ozone. The specific objectives were to determine the extent to which ClO2 preoxidation of the raw water (1) reduced the ozone demand/dose, (2) reduced the bromate formation, (3) reduced the energy consumption, (4) resulted in chlorate ion formation from oxidation of chlorite ion, (5) maintained the filtration benefits of preozonation, and (6) reduced THM and HAA formation potentials. The full-scale demonstration was conducted at the 40 mgd Randall-Bold WTP, which is a direct filtration plant that uses preozonation (before coagulation) to enhance the filtration process and postozonation (after filtration) for primary disinfection. During the study, ClO2 was added to the raw water ahead of the preozonation contactor. The testing component of the study was divided into three phases that led to long-term full-scale operation using ClO2 in combination with ozone. Three preoxidation conditions were tested: 1.5 mg/L of ClO2 alone, the combination of 1.0 mg/L of ClO2 and 0.5 mg/L of ozone, and 1.0 mg/L of ozone alone. In addition, special full-scale testing was conducted to evaluate bromate mitigation techniques such as preoxidation with ClO2, pH adjustment, and ammonia addition.