Sulfate reduction outcompeted methanogenesis at 65 degrees C and pH 7.5 in methanol and sulfate-fed expanded granular sludge bed reactors operated at hydraulic retention times (HRT) of 14 and 3.5 h, both under methanol-limiting and methanol-overloading conditions. After 100 and 50 days for the reactors operated at 14 and 3.5 h, respectively, sulfide production accounted for 80% of the methanol-COD consumed by the sludge. The specific methanogenic activity on methanol of the sludge from a reactor operated at HRTs of down to 3.5 h for a period of 4 months gradually decreased from 0.83 gCOD . gVSS(-1) . day(-1) at the start to a value of less than 0.05 gCOD . gVSS(-1) . day(-1), showing that the relative number of methanogens decreased and eventually became very low. By contrast, the increase of the specific sulfidogenic activity of sludge from 0.22 gCOD . gVSS(-1) . day(-1) to a final value of 1.05 gCOD . gVSS(-1) . day(-1) showed that sulfate reducing bacteria were enriched. Methanol degradation by a methanogenic culture obtained from a reactor by serial dilution of the sludge was inhibited in the presence of vancomycin, indicating that methanogenesis directly from methanol was not important. H-2/CO2 and formate, but not acetate, were degraded to methane in the presence of vancomycin. These results indicated that methanol degradation to methane occurs via the intermediates H-2/CO2 and formate. The high and low specific methanogenic activity of sludge on H-2/CO2 and formate, respectively, indicated that the former substrate probably acts as the main electron donor for the methanogens during methanol degradation. As sulfate reduction in the sludge was also strongly supported by hydrogen, competition between surf ate reducing bacteria and methanogens in the sludge seemed to be mainly for this substrate. Sulfate elimination rates of up to 15 gSO(4)(2-)/L per day were achieved in the reactors. Biomass retention limited the sulfate elimination rate. (C) 2000 John Wiley & Sons, inc.