In this paper the effect of the COD/SO4 ratio on the performance of an anaerobic baffled reactor (ABR) was investigated. Synthetic wastewater (3000 mg-COD/l) was used as the sole organic substrate. ABR was maintained at a hydraulic retention time (HRT) of 1 day and at a temperature of 35 degreesC. The results of this study showed that when COD/SO4 ratios were changed from 16.7 to 6 with increasing sulfate concentration from 180 to 500 mg/l, COD conversion was not decreased, but a slight increase in COD removal was observed. These phenomena indicated that both methanogenic bacteria and sulfate reducing bacteria (SRB) could exist in the system as synthrophism. Maximum COD and sulfate removal were 86 and 97%, respectively, where more than 65% of this removal occurred in first compartment. At high sulfate concentration the most conversion was not happened in the first compartment and therefore shifted to another compartment. (C) 2002 Published by Elsevier Science B.V.