A co-culture of Pseudomonas putida and P. fluorescens immobilized in a fibrous-bed bioreactor was used to degrade benzene, toluene, ethylbenzene and xylenes (collectively known as BTEX), present as sole carbon sources in contaminated water. The kinetics of BTEX biodegradation in the fibrous-bed bioreactor operated under the liquid-continuous condition was studied. Biodegradation rates of BTEX increased with increasing BTEX concentration and reactor loading rate. For benzene, the maximum biodegradation rate was 38 mg/l/h at a loading rate of 265 mg/l/h. For toluene, the rate was 45 mg/l/h at a 100 mg/l/h loading rate. Aeration was not used in the process and the addition of hydrogen peroxide (HO) as an additional oxygen source improved benzene and toluene biodegradation for the high strength synthetic wastewater feeds. When benzene, toluene, ethylbenzene and para-xylene were present as a mixture in the feed, they were concurrently and completely biodegraded under hypoxic conditions (no addition of air or HO). The total BTEX biodegradation rate was as high as 600 mg/l/h at the highest BTEX loading rate, 1000 mg/l/h, studied. Individual BTEX compounds were efficiently and concurrently degraded at a retention time of less than 15 h. Immobilized cells adapted in the bioreactor showed no preferential degradation of BTEX present as mixtures. The bioreactor also had a stable long-term performance, maintaining its ability for efficient BTEX degradation without requiring additional nutrients (e.g. glucose) for more than 1 year. The good performance of the fibrous-bed bioreactor was attributed to the high cell density and unique cell immobilization process provided by the fibrous matrix, which allowed use of the reactor for continued regeneration, adaptation and selection of efficient BTEX degraders in the bioreactor environment. © 2002 Elsevier Science Ltd. All rights reserved.