The main purpose of this study is to assess effects of sulfur compounds and heavy metal on the methanization of tannery wastewater containing high concentration of sulfate and chromium. The volumetric loading rates (VLR) were between 0.8 and 2.0 kgCOD/m(3).d. at hydraulic retention times (HRT) of 18 to 30 h at 35 degrees C. The COD removal efficiency was about 75% during the operation period except temperature shocks. COD removal efficiency and gas production rate decreased rapidly when the temperature dropped to 25 degrees C. In the competition between SRB and MPB, about 13% of the removed COD was utilized by SRB at HRT of 30 h, while about 43% was used by SRB at HRT of 18 h at 35 degrees C. Whereas, after temperature exposure to 25 degrees C about 67% was utilized by SRB at HRT of 26 h. The results indicate that MPB is strongly suppressed by the occurrence of significant sulfate reduction since a large electron flow is distributed to the SRB. In addition, most of trivalent chromium concentration showed removal efficiency above 90%. It is considered that most of chromium concentration accumulated in the reactor probably adsorbs on the sludge or forms hydroxide precipitate. (C) 1997 IAWQ. Published by Elsevier Science Ltd.