The effects of preoxidation with ozone and PEROXONE (hydrogen peroxide and ozone) on in-line filtration performance were examined by conducting pilot-scale experiments and comparing the results with full-scale plant perfor- mance. Preoxidation did not allow for a reduction in coagulant (combined alum and cationic polymer) doses for good treatment yet sometimes caused increased filter run lengths because of lower head loss or delayed turbidity breakthrough. Filter effluent turbidities were similar for no-oxidant and preoxidant trains. Dissolved organic carbon levels in filtered waters were higher for the preoxidation train as compared with no preoxidation. PER- OXONE was moderately more effective than ozone alone in eliminating odors in filtered waters.