Pretreatment of wheat straw with the aqueous ionic liquid. 1-ethyl-3-methylimidazolium acetate, was optimized to maximize fermentable sugars recovery. The optimization process employed a central composite design, where the investigated variables were temperature (130-170 degrees C), time (0.5-5.5 h) and ionic liquid concentration (0-100%). All the tested variables were identified to have significant effects (p < 0.05) on fermentable sugars recovery. The optimum pretreatment conditions were 158 degrees C, an ionic liquid concentration of 49.5% (w/w), and a duration of 3.6 h. Cellulose and xylan digestibility generally increased with increasing temperature, time and ionic liquid concentration; but, the carbohydrates recovered in the washed solids following pretreatment decreased. Thus, the final optimum conditions for maximizing fermentable sugars from the starting biomass were a compromise between greater digestibility and minimal carbohydrates loss during pretreatment. (C) 2011 Elsevier Ltd. All rights reserved.