A toluene degrading microorganism, Pseudomonas putida type A I, was isolated and enriched to treat toluene vapor in a hollow fiber membrane bioreactor. Prior to the membrane reactor experiments, a set of batch experiments using this microorganism was conducted to examine the effect of oxygen levels on the microbial activity. It was found that a low oxygen level does limit the degradation rate of toluene. A hydrophobic hollow fiber module was developed for biological degradation of toluene vapor with enhanced oxygen transfer capabilities. A hydrophobic polyethylene microfiltration hollow fiber membrane bioreactor showed that toluene removal efficiencies were consistently kept in the range of 86-97% at the loads of 0.85-4.3 kg Tol./m(3) day for 150 days. The result showed that a hollow fiber membrane bioreactor system would be a useful choice for treating air emissions containing volatile organic compounds (VOC). (c) 2004 Elsevier Ltd. All rights reserved.