Four organosolv pretreatment processes, namely Formiline, Acetoline, sulfuric acid-catalyzed ethanol (SACE) and auto-catalyzed ethanol (ACE) pretreatments were used to pretreat wheat straw under respective optimal conditions. Comparative studies were performed in terms of the enzymatic digestibility of cellulosic solids, simultaneous saccharification and fermentation (SSF) for ethanol production, mass balance analysis and structural features of the pretreated solids. The results indicated that Formiline and Acetoline pretreatments showed higher degrees of delignification, lower xylose degradation and lower solid glucan recoveries than SACE and ACE processes. The SACE and ACE pretreated solids still had high lignin contents but demonstrated higher initial enzymatic hydrolysis rates than Formiline and Acetoline pretreated solids. Formiline pretreatment obtained the highest final enzymatic glucan conversion, glucose concentration and ethanol yield, while Acetoline pretreatment obtained the lowest. After organosolv pretreatment, the compact and beehive-like cell wall structure was disrupted and cellulose fibers became separated. The SACE and ACE pretreated solids showed more depositions on fiber surface, which probably came from the condensation and precipitation of lignin. (C) 2015 Elsevier Ltd. All rights reserved.