Reduced xylose fermentation performance has been an issue during fermentation of AFEX(TM) hydrolysate using Saccharomyces cerevisiae 424A (LNH-ST) or Escherichia coli KO11. To better understand why fermentation performance is reduced, we quantitatively studied the effects of compounds present in the fermentation broth on xylose consumption. The compounds include biomass degradation products, ethanol and fermentation metabolites. The xylose consumption capability of E. coli KO11 was almost totally inhibited by the presence of both degradation products and ethanol. On the other hand, for S. cerevisiae 424A, 89% reduction of xylose consumption rate was found during hydrolysate fermentation. Degradation products, ethanol and fermentation metabolites were responsible for 32%, 24% and 33% of such reduction, respectively. Those results suggest that to further improve the xylose fermentation in hydrolysate, strains should be selected not only for degradation products tolerance but also for ethanol and fermentation metabolites tolerance. (C) 2012 Elsevier Ltd. All rights reserved.