The TiO2/UV/O-3 process has been employed to remove 4-chloronitrobenzene (4-CNB) and compared to UV/air, O-3,TiO2/O-3,TiO2/UV/O-2 and UV/O-3 five parallel oxidation pathways. The reaction activities of these six processes were tested in aqueous using electron paramagnetic resonance (EPR) spin trapping technique with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap. Moreover, the effects of ozone dosage, catalyst dosage and initial solution pH on the degradation of 4-CNB by TiO2/UV/O-3 process were also investigated. Results showed that the TiO2/UV/O-3 is the most efficient process for complete mineralization of 4-CNB since the combination of photocatalytic oxidation with ozone has a synergistic effect. The relative intensity of DMPO-(OH)-O-center dot follows the order of UV/air < O-3 < TiO2/O-3 < UV/O-3 < TiO2/UV/O-2 < TiO2/UV/O-3. In TiO2/UV/O-3 process, the ozone and catalyst dosage are found to have a significant impact on the process efficiency whereas initial pH has relatively less effect. Chlorine atoms of 4-CNB are absolutely oxidized to chloride anions when the ozone dosage in the range of 5-18 mg/min, or the catalyst dosage ranging from 1.0 to 2.0 g/L. Part of chloride anions would be further oxidized to chlorate(V) ions if the ozone dosage exceeds 24 mg/min, or the catalyst dosage exceeds 2.0 g/L. (C) 2009 Elsevier B.V. All rights reserved.