The effect of the combination of acetic acid, furfural and catechol ethanol fermentation of Kluyveromyces marxianus was studied. A modified central composite design was used to investigate the influence of these compounds on specific growth rate. ethanol yield and biomass yield. The compounds were selected taking into account their concentration in the liquid fraction from the pretreatment and their toxicity. According to the model applied furfural had a great negative effect on specific growth rate, whereas ethanol yield and biomass yield were more affected by the interaction among the tested compounds. Acetic acid, furfural and catechol showed a synergistic effect on toxicity with respect to ethanol yield and biomass yield. (c) 2005 Elsevier Ltd. All rights reserved.