The effects of temperature and initial pH on anaerobic xylose utilization by the thermotolerant yeast Kluyveromyces marxianus IMB2, IMB4, and IMB5 were evaluated. Ethanol yield from xylose was greatest with IMB4, a temperature of 40 degrees C and an initial pH of 5.5. Xylitol yield was greatest with yeast strains IMB2 and IMB5, a temperature of 45 degrees C and an initial pH of 4.5 or 5.0. Additional fermentations of xylose at 40 and 45 degrees C, pH 5.5, and 20 times more cells were done using IMB4 since it exhibited the greatest ethanol yield. Ethanol and xylitol yields from xylose were greater at 40 degrees C than at 45 degrees C and volumetric production rates for ethanol and xylitol were 0.02 and 0.08 g/(1 h), respectively. IMB4 xylitol yields were comparable to other xylitol producers, but with lower productivity. These strains are not good candidates for utilization of xylose under anaerobic conditions and other strategies should be explored. (C) 2007 Elsevier Ltd. All rights reserved.