||In anaerobic treatment of sulfate-laden organic wastewater, sulfate is reduced to sulfide by sulfate-reducing bacteria. This reduction is undesirable as it reduces the methane yield by substrate competition and/or sulfide toxicity on methanogenic bacteria. Moreover, the fuel value of biogas is reduced by the presence of hydrogen sulfide, which forms sulfuric acid upon combustion and causes corrosion of plant machinery. This study investigated the effect of oxidation-reduction potential (ORP) on the competition between sulfate-reducing bacteria and methanogens. The ultimate aim was to evaluate the feasibility of developing a two-stage anaerobic system to treat the sulfate-laden wastewater. That is, the first stage is used to allow only sulfate reducers to function through a proper ORP control, and the second stage for methanogenesis. To achieve this goal, experimental tests had been carried out to evaluate the responses of both sulfate reducers and methanogens at different ORP with different combinations of COD/SO4-2 ratios (with a constant influent COD of 10,000 mg/l or TOC of 3750 mg/l). At the natural operating ORP of around -285 mV, temperature 35°C and HRT of 15 days, the activities of the sulfate reducers were not affected by the influent sulfate increase from 1,000 to 5,000 mg/l. However. The methanogenic activity was progressively reduced by the increase of influent sulfate concentration. More specifically, the methane production rate was 176 ml/L-day at 1,000 mg/l of sulfate, and the value was reduced to 108 and 96 ml/L-day, respectively, when the sulfate concentration was increased to 3,000 and 5,000 mg/l. When the operating ORP was increased to -235 mV, the sulfate-reducing activities were not adversely affected as long as the influent sulfate concentration did not exceed 3,000 mg/l. But as the influent sulfate was increased to 5,000 mg/l, the sulfate reducing bacteria became inhibited. The level of sulfate reducers' inhibition exceeded that of methanogens at this ORP. As such, the methanogens became more competitive for the available organic carbon. This caused a substantial increase of the methane yield under such a specific operating condition due to the elimination of substrate competition from the sulfate reducers. When the ORP was further increased to -185 mV, the methanogenic activity became almost totally inhibited while the sulfate reducers were not. Thus, for treating high-strength sulfate-laden wastewater, it seems feasible to control the operating ORP at -185 mV to arrest the meekandgenie activity, yet still allowing the sulfate reducers to remove sulfate with full potential without inhibition.