Hydrogen gas attracts great interest as a potential clean future fuel and it is an excellent electron donor in biotechnological reductive processes, e.g. in biodesulfurization. Bulk production of H 2 relies on the conversion of organic matter into synthesis gas, a mixture of H 2 , CO and CO 2 . The relative abundance of CO restricts its applicability, due to toxicity to hydrogenotrophic microorganisms and poisoning of chemical catalysts in low temperature fuel cells. Currently, synthesis gas purification, i.e. CO conversion to H 2 , is performed in chemical catalytic systems. A recently discovered group of thermophilic anaerobic bacteria is able to grow by converting CO with water to H 2 and CO 2 . This feature makes thesehydrogenogensinteresting for cost effective hydrogen production.Several anaerobicwastewatertreatingsludgesharbor CO utilizing moderately thermophilic (55°C) hydrogenogenic microorganisms. CO conversion at 30°C resulted in the production of acetate, whereas at 55°C it proceeded via H 2 /CO 2 .One of the tested sludge samples could even reduce sulfate with the CO-derived H 2 , tolerating and using high CO concentrations (P CO >160kPa). From this sludge amoderately thermophilic, anaerobic, sulfate-reducing bacterium was isolated, i.e.Desulfotomaculumcarboxydivorans , capable of growth on CO as sole energy and carbon source both in the presence and absence of sulfate as electron acceptor. D. carboxydivorans grows rapidly at 200kPaCO, pH 7.0 and 55ºC (t d of 100 minutes), producing nearlyequimolaramounts of H 2 and CO 2 from CO revealing a high specific CO conversion rate of 0.8 mol CO.(g protein) -1 .hour -1 . Furthermore, D. carboxydivorans is capable of hydrogenotrophic sulfate reduction at partial CO pressures exceeding 100kPa, at a maximal specific sulfate reduction rate of 32mmol.(g protein) -1 .hour -1 . These characteristics make it an interesting candidate for synthesis gas purification as well as for the direct use of synthesis gas in biodesulfurization at elevated temperatures. Although in the latter case, the low sulfide tolerance of D. carboxydivorans , i.e. total inhibition at 5mMand 9mMat pH 6.5 and 7.2, respectively, may require special features to maintain sufficient low sulfide concentrations.Thermophilicsulfate reduction using CO as electron donor with anaerobic granular sludge, from which D. carboxydivorans originated, showed that despite the high CO conversion capacity of the biomass present, the sulfate reduction capacity was limited due to strong competition for the produced H 2 . Operation at HRT >9 hours resulted in a predominant consumption of the CO-derived H 2 by methanogens (up to 90%) and thus in a poor sulfate reduction efficiency (<15%). Although, the methanogens appeared to be more sensitive to pH and temperature shocks imposed to the reactor, they were not eliminated by these treatments. The high growth rates of the methanogens (t d of 4.5 hours) resulted in fast recovery and domination of the consumption of CO-derived H 2 by methanogens. At HRT <4 hours, the consumption of CO-derived H 2 was dominated by the sulfate reducing bacteria (up to 95%). The highest sulfate reduction rates achieved were17 mmol.L -1 .day -1 at a HRT of 3 hours (87% of the H 2 used by sulfate reducers). These rates were limited by the amount of CO supplied and the CO conversion efficiency (85%) at higher CO loads (106 mmol.L -1 .day -1 ), probably as a result of limited biomass retention in the reactor.Elimination of methanogenesis is a prerequisite for practical application of both synthesis gas utilization as electron donor for thermophilic sulfate reduction processes and synthesis gas purification. Short-term (90 minutes) pretreatment of the sludge at95°Celiminatesmethanogenesis, but nothomoacetogenesis. Although, homoacetogens did not seem to reduce the electron flow towards sulfate reduction much, their activity represents an unwanted loss of H 2 . For practical applications a complete heat-treatment of the sludge at temperatures exceeding at least 85ºC and treatment of the (empty) reactor system using steam as well as additional measures to prevent introduction of amethanogenicpopulation could be considered.