The investigation of the performance of UV/H2 O2 , UV/O3 and UV/H2 O2/O3 oxidation systems treating spent caustic from an ethylene plant shows that in UV/H2 O2 system, with the increase of H2 O2 dosage, removal efficiencies of COD and the ratio of biochemical oxygen demand (BOD) to chemical oxygen demand (COD) of the effluent were increased and a better performance was obtained than the H2O2system alone. In UV/H2O2 system, removal efficiency of COD reached 68% under the optimum condition, and BOD/COD ratio was significantly increased from 0. 22 to 0. 52. In UV/O3 system, with the increase of O3dosage, removal efficiency of COD and BOD/COD ratio were increased, and a better performance was obtained than the O3 system alone. Under the optimum condition, removal efficiency of COD was 54%, and BOD/COD ratio was significantly increased from 0. 22 to 0. 48. In UV/H2O2/O3 system, COD removal efficiency was found to be 22.0%higher than UV/O3 system.