Biologically treated incineration leachate contains a high amount of refractory organic matter such as humic-like and fulvic-like acids. Due to the high concentration of organic matter (390-500 mg L-1 of chemical oxygen demand (COD)) and low biodegradability, this wastewater must be chemically treated. A combined vacuumultraviolet (VUV) with O-3 process was employed to degrade COD and improve the biodegradability of wastewater. The optimal treatment conditions were: 11.2 mg min(-1) of O-3 dosage, initial pH of 9, and a 60-min treatment time. At optimal conditions, BOD5/COD increased significantly from 0.16 to 0.73. Both humic-like and fulvic-like substances were degraded into aliphatic intermediates with low molecular weights. In the VUV/O-3 system, VUV irradiation sharply accelerated O-3 utilization efficiency and center dot OH accumulated. center dot OOH, center dot H, and center dot OH were detected as the main free radicals for the degradation of refractory organic compounds by the VUV/O-3 system.