Acetone-based organosolv fractionation of wheat straw was studied to produce sugars and lignin in a lignocellulose biorefinery. The influence of acetone-water ratio, reaction time, and temperature on fractionation degree, lignin yield, and enzymatic digestibility of the cellulose fraction was examined. The solvent water ratio was found to influence delignification and hemicellulose hydrolysis by its effect on lignin solubility and pH. Organosolv treatment in 50:50% w/w acetone-water during 1 h at 205 degrees C resulted in 82% hemicellulose hydrolysis, 79% delignification, and 93% cellulose recovery. A substantial part of the sugars formed upon hemicellulose hydrolysis seems to have reacted further including possible formation of lignin-furfural condensation products. Finally, aqueous acetone pretreatment was found to substantially improve the enzymatic digestibility of the cellulose fraction (glucose yield: up to 87% vs 16% for untreated wheat straw).