In this study, we employed stepwise dilute sulfuric acid-catalyzed hydrothermal pretreatment and alkaline fractionation to enhance digestion of triploid poplar for bioconversion. Samples of triploid poplar were subjected to a pretreatment with 0.5% sulfuric acid at different temperatures and then to fractionation with 70% aqueous ethanol solution containing 1.5% NaOH. The results indicated that the stepwise pretreatment process degraded hemicelluloses, incurring slightly increase in crystallinity of cellulosic residues. Lignin was concentrated during acidic pretreatment and negatively affected the interaction between enzyme and cellulose. As the pretreatment temperature increased to 200 degrees C. the cellulose was degraded and exhibited lower crystallinity. The removal of polysaccharides and lignin resulted in mass loss and considerable feedstock recoveries were achieved at the temperatures below 130 degrees C. The results obtained from enzymatic hydrolysis suggested that the stepwise pretreatment enhanced the digestibility of the cellulosic residues. The optimum pretreatment temperature was observed at 120 degrees C. In this case 60.3% lignocellulose was recovered and achieved 69.4% of cellulose-to-glucose in enzyme-mediate conversion. (C) 2012 Elsevier Ltd. All rights reserved.