Biological sulfate (SO 4 ) reduction with carbon monoxide (CO) as electron donor was investigated. Four thermophilic SO 4 -reducing bacteria, Desulfotomaculum thermoacetoxidans (DSM 5813), Thermodesulfovibrio yellowstonii (ATCC 51303), Desulfotomaculum kuznetsovii (DSM 6115; VKM B-1805), and Desulfotomaculum thermobenzoicum subsp. thermosyntrophicum (DSM 14055), were studied in pure culture and in co-culture with the thermophilic carboxydotrophic bacterium Carboxydothermus hydrogenoformans (DSM 6008). D. thermoacetoxidans and T. yellowstonii were extremely sensitive to CO: their growth on pyruvate was completely inhibited at CO concentrations above 2% in the gas phase. D. kuznetsovii and D. thermobenzoicum subsp. thermosyntrophicum were less sensitive to CO. In pure culture, D. kuznetsovii and D. thermobenzoicum subsp. thermosyntrophicum were able to grow on CO as the only electron donor and, in particular in the presence of hydrogen/carbon dioxide, at CO concentrations as high as 50-70%. The latter SO 4 reducers coupled CO oxidation to SO 4 reduction, but a large part of the CO was converted to acetate. In co-culture with C. hydrogenoformans, D. kuznetsovii and D. thermobenzoicum subsp. thermosyntrophicum could even grow with 100% CO (P CO =120 kPa).