Biotechnological conversion of xylose into xylitol using hydrolysates obtained from the hemicellulosic fraction of lignocellulosic materials is compromised by the presence of compounds released or formed during the hydrolysis process, some of them being toxic to microorganisms. In order to improve the bioconversion of these hydrolysates it is necessary to find methods to reduce their toxicity. In the present work, rice straw hemicellulosic hydrolysate was treated by six different procedures (all of them involving pH adjustment, with or without activated charcoal adsorption), before being used as a fermentation medium for xylitol production. The most effective method of treatment was to increase the initial pH (0.4) to 2.0 using solid NaOH, followed by the addition of activated charcoal (25 g kg(-1)) and increase in the pH to 6.5 using solid NaOH. Lignin degradation products were the most inhibitory compounds present in the hydrolysate; their removal was selective and strongly dependent on the pH employed in the treatment. The highest yield of xylitol was 0.72 g g(-1) xylose, with a productivity of 0.55 g dm(-3) h(-1). (C) 2004 Society of Chemical Industry.