Combined biohydrogen and bioethanol (CHE) production from xylose was achieved by an extreme thermophilic (70 degrees C) mixed culture. Effect of initial pH, xylose, peptone, FeSO(4), NaHCO(3), yeast extract, trace mineral salts, vitamins, and phosphate buffer concentrations on bioethanol and biohydrogen yield was investigated in batch experiments. Results obtained showed that initial pH, concentration of xylose, peptone, and FeSO(4) significantly affected biohydrogen and bioethanol production, while the concentration of NaHCO(3) was only significant for bioethanol production. By changing cultivation conditions the culture could be directed to mainly produce ethanol with maximum ethanol yield of 1.60 mol ethanol/mol-xylose corresponding to 95.8% of the theoretical ethanol yield based on degradation of xylose through ethanologenic pathway, or mainly hydrogen with maximum hydrogen yield of 1.84 mol H(2)/mol-xylose corresponding to 55% of the theoretical hydrogen yield based on acetate metabolic pathway. An empirical model was established to reveal the quantitative effect of factors significant for biohydrogen (quadratic model) production and for bioethanol (linear model) production. Changes in hydrogen/ethanol yields observed were due to the shift of the metabolic pathway between ethanol or hydrogen production, rather than changes in bacterial community composition at genus level. Thermoanaerobacter related bacteria were found to be the dominant hydrogen/ethanol producers. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.