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Differences in the relative toxicity of xylose-rich prehydrolysates derived from woody and herbaceous feedstocks are likely due to the relative abundance of a variety of inhibitory compounds. Acetate, as well as several aromatic monomers, has been shown to be an inhibitor of the xylose-fermenting yeast, Pichia stipitis. Comparative information on the concentration of known and likely inhibitors, other than acetate, is lacking. The present study provides data on the aromatic monomer composition of representative herbaceous and woody prehydrolysates. Dilute-acid prehydrolysates were prepared from three feedstocks; two herbaceous, corn stover and switchgrass (Panicum virgaturn L.), and one woody (poplar). The prehydrolysates were neutralized with Ca(OH)[sub 2], extracted with ethyl acetate, trimethylsilylated, and analyzed by GC-MS. Fourteen aromatic monomers were tentatively identified by comparison with published mass spectra. The concentrations of the aromatic monomers totalled 112, 141 and 247 mg L[sup -1] for corn stover, switchgrass and poplar prehydrolysates, respectively. This is also the order of increasing inhibition of growth and ethanol productivity observed for Pichia fermentations. The woody prehydrolysate contained approximately four-fold more syringyl-based monomers than did the herbaceous prehydrolysates, while guaiacyl-containing compounds were more evenly distributed. [ABSTRACT FROM AUTHOR]