A detoxification method using activated charcoal with concentrated rice straw hemicellulosic hydrolysate improved the conversion of xylose to xylitol by the yeast Candida guilliermondii by 22%. This was achieved when the hydrolysate:charcoal ratio was 40 g g(-1), resulting in removal of 27% of phenolic compounds. Under this condition, the xylitol yield factor (0.72 g g(-1)) and volumetric productivity (0.61 g l(-1) h(-1)) were close to those attained in a semi-defined medium simulating hydrolysate sugars.