The Contra Costa Water District (CCWD), through an American Water Works Association Research Foundation (AwwaRF) Tailored Collaboration Project, conducted an evaluation of the efficacy of preoxidation with a combination of chlorine dioxide (ClO2) and ozone (O3) at its 40 mgd Randall-Bold Water Treatment Plant (WTP). This facility is a direct filtration plant that uses raw water preozonation for taste and odor control and filtration enhancement and ozonation after filtration (post ozonation) for primary disinfection. Results indicated that: ClO2 addition before preozonation reduced bromate formation; a majority of the chlorite formed by ClO2 reactions with raw water constituents was oxidized to chlorate by ozonation; and, preoxidation with ClO2 was as effective as preoxidation with ozone for enhancing filtration.