The ethanol and hydrogen production capacity from grass and cellulose hydrolysates (HL) by Thermoanaerobacter J1 and Thermoanaerobacter J4 was studied in batch cultures. Strain J1 was most closely related to T. ethanolicus (97.4%) and strain J4 T. thermohydrosulfuricus (98.2%). Strain J1 is highly ethanologenic strain, producing 1.7 mol ethanol form one mole of glucose but strain J4 is a good hydrogen producer with 3.1 mol H2 from one mole of glucose. Two electron scavenging systems were used during growth of the strains on glucose. Thiosulfate (40 mM) was added and the strains were co-cultured with a hydrogenotrophic methanogens. This resulted in a shift in end product formation for strain J1 but increased acetate formation for strain J4. Cultivation of both strains on grass hydrolysates was done by using three different HL concentration and three different liquid-gas ratios. Strain J1 degraded all sugars present in the HL, and the ratio of end product remained the same. Strain J4 was however severely inhibited by increasing the substrate loading and was hugely affected by different liquid gas phases used, thus producing most acetate and hydrogen at the lowest liquid gas phase ratio. Strain J1 produced maximum of 4.7 mol ethanol/g grass HL and strain J4 a maximum of 5.7 mol H₂/g grass HL.