Two hollow fiber membrane bioreactors (HFMBRs) inoculated with activated sludge were used in series to biodegrade continuously mixed xylene. The influence of gas residence time (tau) and mass loading rate (LR) on elimination capacity (EC) of the mixed xylene was investigated. A maximum elimination capacity (ECmax,v) of 466 g m(-3) h(-1) was achieved at tau = 10 s and LRv = 728 g m(-3) h(-1). Thereafter, ozone was introduced into inlet gas and the influence of ozone was investigated. Results showed that the maximum xylene elimination capacity increased from 524 g m(-3) h(-1) to 568 g m(-3) h(-1) and 616 g m(-3) h(-1) at tau = 10 s, respectively when the inlet ozone concentration rose from 200 mg m(-3) to 400 mg m(-3) and 600 mg m(-3), respectively. HFMBR coupled with O-3 has higher performance and stability for the long-term operation at the same condition. (C) 2012 Elsevier Ltd. All rights reserved.