A denitrifying consortium was used as inoculum to form a biofilm in horizontal-flow anaerobic immobilized biomass reactor. The reactor was fed with hydrocarbons, separately (benzene at 13.8, 15.4 and 26.5 mg; toluene 30.8 mg/L; ethylbenzene 33.3 mg/L; xylene 32.1 mg/L), and also with a benzene, toluene, ethylbenzene and xylene (BTEX) mix solution of approximately 5.0 ing/L. of each hydrocarbon. The hydrocarbons were dissolved in a solution containing ethanol. Organic matter removal efficiencies were of 95% with benzene and toluene amendments and about 76% with ethylbenzene, m-xylene and the BTEX-mix amendments. Hydrocarbons removal efficiencies were of 99% at an initial concentration of benzene 26.5 mg/L, toluene 30.8 mg/L, m-xylene 32.1 mg, ethylbenzene 33.3 mg/L and BTEX 26.5 mg/L. Microbial diversity assessed by a small portion of 16S DNA suggested the predominance of species related to the phylotypes Pseudomonas, Paracoccus and Bacteroides. This system showed to be an alternative to treating wastewater contaminated with nitrate, ethanol and hydrocarbons. (c) 2006 Elsevier Ltd. All rights reserved.