The contribution of fermentative bacteria and methanogenic archaea to azo dye reduction by a thermophilic anaerobic consortium was studied, Additionally, the effects of different electron-donating substrates and the redox mediator riboflavin on dye reduction were assessed by using either a methanogenic consortium or pure cultures of methanogens. Our results indicate that fermentative bacteria and methanogenic archaea play an important role in this reductive process. The thermophilic methanogens Methanothermobacter thermoautotrophicus Delta H and a Methanothermobacter-related strain NJ1 were only able to reduce the dye in the presence of riboflavin. This suggests that anaerobic dye reduction is not a universal property among methanogenic archaea and that redox mediators may improve reductive decoulorisations by allowing some microbial groups commonly found in wastewater treatment systems to participate more effectively. (c) 2005 Elsevier Inc. All rights reserved.