The effect of inhibitors on the recombinant xylose fermenting strain Saccharomyces cerevisiae 259ST was compared to three reference strains, including the parent strain (S. cerevisiae 259A) and an industrial strain adapted to spent sulfite liquor (SSL). Interaction effects between the inhibitors were verified by a fractional factorial design. S. cerevisiae 259ST was found to be as hardy as the reference strains towards acetic acid, ammonium, furfural, and osmotic effects, which are inhibitory compounds in SSL. Ammonium toxicity appeared to be due to osmotic effects. For all of the inhibitors tested., growth rate was more severely inhibited than the ethanol yield during fermentation of glucose. The ability of S. cerevisiae 259ST to ferment xylose was more severely affected than the ability to ferment glucose. At the concentrations expected in softwood SSL (pH 5), acetic acid will decrease the growth rate by 15%, while the ethanol yield on xylose would decrease by 50%, while ammonium will decrease the yeast growth rate by 20% and decrease the ethanol yield from xylose by 45%. (C) 2003 Elsevier Inc. All rights reserved.