Biomass forms a complex interwoven structure containing cellulose, hemicellulose and lignin that hinders enzymatic hydrolysis of cellulose. Enzymatic hydrolysis of the cellulose within yellow poplar (tulip tree) particles released only 9% of the total glucose in this study. To increase the accessibility of the cellulose component, wood particles were pretreated using hot compressed water and enzymatically-generated peracetic acid. The combined pretreatment started with hot compressed water (200 degrees C, 15 min), which selectively solubilized up to 90% of the xylan. The remaining solid was treated with peracetic acid (90 mM, 60 degrees C, 6 h), which solubilized up to 70% of the lignin. The remaining solid consisted of mainly glucan (similar to 75%) and corresponds to 87% of the glucan initially present in the yellow poplar particles. Hydrolysis of the remaining solid using a low loading of cellulase/beta-glucosidase for 72 h released 90% of the glucose. The removal of the xylan and lignin structural barriers dramatically increased the cellulase accessibility to cellulose. The structural characteristics (crystallinity, functional group changes, morphology) of combined pretreated solid residue changed in a manner consistent with increased enzymatic digestibility. The combined pretreatment with hot compressed water and peracetic acid was more effective than either single pretreatment and more effective than the sum of the single pretreatments to remove xylan and lignin, thus demonstrating a cooperative effect of the two pretreatments. In addition, the combined pretreatment enhanced the accessibility of cellulases to the cellulose resulting in more efficient cellulose hydrolysis. (C) 2017 Elsevier Ltd. All rights reserved.