Two bench-scale horizontal-flow anaerobic immobilized biomass (HAIB) reactors were assayed with the purpose of evaluating their performance in removing gasoline from contaminated groundwater. The first reactor was fed initially with synthetic substrate containing protein, carbohydrates and benzene, toluene, ethylbenzene, and xylene (BTEX) solubilized in ethanol. The BTEX concentration of each compound ranged from 3.0 to 15.0 mg/l. The BTEX solution in ethanol was then used as the sole carbon source. The hydraulic retention time (HRT) was 11.4 h and the temperature was kept constant at 30 +/- 1 degreesC. Chemical oxygen demand (COD) removal efficiencies of about 96% and BTEX removal efficiencies varying from 75 to 99% were achieved during this experimental period. The second reactor was operated to simulate actual field conditions. Thus, the HAIB reactor was fed with a mixture of water and commercial gasoline devoid of ethylbenzene. The concentration of each compound considered in BTX composition (benzene, toluene, and m-xylene) was gradually raised to 15.0 mg/l. The HRT was 20.0 h and the temperature was kept at 25 +/- 1 degreesC. The HRT was progressively lowered to 8.0 h to investigate the reactor loading rate capacity. The best COD and BTX removal efficiencies achieved were about 99% and 95%, respectively, with a HRT exceeding 12 h. The results indicate the viability of using the HAIB reactor to treat wastewater and groundwater contaminated with BTEX and gasoline. (C) 2004 Elsevier Ltd. All rights reserved.