This work assessed the anaerobic BTEX removal performance of a sulfate-reducing bioreactor at different COD/SO42- ratios and co-substrate (ethanol) concentrations. Additionally, the effect of the different operational conditions on the structure (diversity, evenness and richness) of the bacterial and archaeal communities was investigated. The addition of sulfate at different COD/SO42- ratios did not change BTEX removal (60-67%). However, a likely inhibition of COD removal was observed for COD/SO42- ratios below 5 (72-81%). Although, initially, ethanol concentration reduction seemed to enhance BTEX removal (from 60% to 73%), especially for benzene (46%) and toluene (76%), the results about the actual impact of co-substrate shortage were inconclusive, even though the statistical tests indicated that ethanol affected negatively BTEX removal. The optimal degree of evenness likely contributed to the high stability of the system in terms of BTEX removal. Changes observed in bacterial and archaeal richness did not match with the functioning of the system. However, dynamics and evenness parameters seemed to be of importance in maintaining a stable reactor performance. (C) 2014 Elsevier B.V. All rights reserved.