A thermophilic (55 degreesC) lab-scale (0.921) methanol-fed upflow anaerobic sludge bed (UASB) reactor (pH 7.0 and hydraulic retention time (HRT) of 7.5 h) was operated at chemical oxygen demand (COD) to sulfate (SO42-) ratios of 10, 5 and 0.5 during 155 days to evaluate the effects of the presence of sulfate on conversion rates, metabolic shifts and possible process disturbances. Methanol was completely removed when operating at an organic loading rate of 20 g COD l(-1) day(-1) at all COD/SO42- ratios tested. At COD/SO42- ratios of 10 and 5. methanol was converted both via sulfate reduction (up to 13% when operating at a COD/SO42- of 5) and methanogenesis (85%). However, when operating at a COD/sulfate ratio of 0.5 (12 g SO42- l(-1)), the sulfate reduction efficiency strongly deteriorated, due to improper immobilization of sulfate reducing bacteria (SRB) in the sludge bed and the presence of relatively high sodium concentrations (about 6 g Na+ l(-1)) originating from supplying sulfate as its sodium salt. Complete sulfate reduction was achieved when operating at a COD/SO42-ratio of 10 (0.6 g SO42- l(-1)) and 5 (1.2 g SO42- l(-1)), corresponding to sulfate removal rates of 2 and 4 g SO42- l(-1) day(-1), respectively. Activity tests showed that methanol was syntrophically converted via H-2/CO2 by homoacetogenic bacteria, in combination with either sulfate reducing bacteria or methane producing archaea. (C) 2003 Elsevier Science Inc. All rights reserved.