The effect of lignin degradation products liberated during chemical hydrolysis of lignocellulosic materials on xylose-to-xylitol bioconversion by Candida guilliermondii FTI 20037 was studied. Two aromatic aldehydes (vanillin and syringaldehyde) were selected as model compounds. A two-level factorial design was employed to evaluate the effects of pH (5.5–7.0), cell concentration (1.0–3.0 g l −1), vanillin concentration (0–2.0 g l −1) and syringaldehyde concentration (0–2.0 g l −1) on this bioprocess. The results showed that in the presence of vanillin or syringaldehyde (up to 2.0 g l −1) the cell growth was inhibited to different degrees with a complete inhibition of the yeast growth when the mixture of both (at 2.0 g l −1 each) was added to the fermentation medium. The xylitol yield was not significantly influenced by vanillin, but was strongly reduced by syringaldehyde, which showed a more pronounced inhibitor effect at pH 7.0. The yeast was also able to convert vanillin and syringaldehyde to the corresponding aromatic acids or alcohols and their formation was dependent of the experimental conditions employed.