An effective bioconversion of xylose into xylitol by Candida guilliermondii FTI 20037 requires sugarcane bagasse hydrolyzate with a high level of xylose and a low concentrations of toxic compounds. The composition of the hydrolyzate depends on the vacuum evaporation process variables (pH, temperature, xylose concentration degree and treatment with activated charcoal before or after the process). The influence of these variables on xylitol volumetric productivity was measured using a 2(5-1) fractional factorial central composite design followed by a response surface methodology. The model predicted a xylitol volumetric productivity of 0.350 g/l h in hydrolyzate treated with charcoal (pH 0.92) and subsequently evaporated under vacuum to reach a xylose concentration of 51.0 g/l. Under these conditions, the xylose, acetic acid and arabinose consumption levels were 93, 57 and 13.62%, respectively, and xylitol yield and production were 0.54 g/g and 24.33 g/l, respectively. The final cell concentration was fivefold, the initial value (0.278 x 10(8) cell/ml). The temperature levels (43-80 degreesC) tested during vacuum evaporation did not influence xylitol production significantly. (C) 2002 Elsevier Science Ltd. All rights reserved.