Biohydrogen production from xylose by Thermotoga neapolitana was investigated in batch culture using serum bottles and a continuously stirred anaerobic bioreactor (CSABR). The effect of various xylose concentrations on growth and H(2) production were studied in small batch culture for highly efficient H(2) production. The highest hydrogen production of 32.1 +/- 1.6 mmol-H(2)/L and maximum biomass concentration of 959.63 +/- 47.9 mg/L were obtained at initial xylose concentration of 5.0 g/L To develop a large-scale biohydrogen production system as well as overcome the problems in small batch culture, a continuously stirred anaerobic bioreactor was tested on T neapolitana in both pH-uncontrolled batch culture and pH-controlled batch culture. The results showed that the production level of H(2) from fermentation in a pH-controlled batch culture was much higher than those from a pH-uncontrolled batch culture for H(2) production from xylose. The H(2) yield in a pH-controlled batch culture on xylose substrate was 2.22 +/- 0.11 mol-H(2) mol(-1) xylose(consumed), which was nearly 1.2-fold higher than pH-uncontrolled batch cultures. In order to study the precise effect of a stable pH on hydrogen production, and metabolite pathway involved, cultures was conducted with pH-controlled at different levels ranging from 6.5 to 7.5. The maximum H(2) yield of 2.8 +/- 0.14 mol-H(2) mol(-1) xylose(consumed) was measured while the pH was maintained at 7.0. The acetic acid and lactic acid production were 2.98 +/- 0.15 g/L and 0.36 +/- 0.02 g/L, respectively. (C) 2011 Elsevier Ltd. All rights reserved.