Semidefined media fermentation simulating the sugar composition of hemicellulosic hydrolysates (around 85 g l(-1) xylose, 17 g l(-1) glucose, and 9 g l(-1) arabinose) was investigated to evaluate the glucose and arabinose influence on xylose-to-xylitol bioconversion by Candida guilliermondii. The results revealed that glucose reduced the xylose consumption rate by 30%. Arabinose did not affect the xylose consumption but its utilization by the yeast was fully repressed by both glucose and xylose sugars. Arabinose was only consumed when it was used as a single carbon source. Xylitol production was best when glucose was not present in the fermentation medium. On the other hand, the arabinose favored the xylitol yield (which attained 0.74 g g(-1) xylose consumed) and it did not interfere with xylitol volumetric productivity (Q(P) = 0.85 g g(-1)), the value of which was similar to that obtained with xylose alone.