Zymomonas mobilis CP4(pZB5) is a recombinant bacterium that can produce ethanol from both xylose and glucose. The ethanol-producing efficiency of this organism is substantially impeded by toxic substances present in pretreated hydrolyzates or solid biomass substrates. Acetic acid and furfural (a pentose degradation product) are highly toxic to this organism at levels envisioned for a pretreated-hardwood liquid hydrolyzate. in addition, lignin degradation products and 5-hydroxymethylfurfural (a hexose degradation product) have a moderately toxic effect on the organism. Of the compounds studied, organic acids and aldehydes were found to be more inhibitory than lignin acids or the one alkaloid studied. Acetone:water and methanol extracts of solid biomass samples from red oak, white oak, and yellow poplar are toxic to Zymomonas cell growth and ethanol production, with the extracts from white oak being the most toxic.