Ionic liquids (ILs) have been widely used as alternative solvents for biomass pretreatment, however, efficient methods that enable economically use of ILs at large scale have not been established. In this study, a new method in which ILs and polar organic solvents (ILs/co-solvent systems) was proposed for efficient pretreatment of lignocellulosic materials. The combination use of appropriate ILs and organic co-solvents can significantly enhance the solubility of lignocellulose due to the lower viscosity of ILs/co-solvent mixture as compared to those of pure ILs while the hydrogen bond basicity was maintained. In addition, the solubility of lignocellulosic materials in ILs/co-solvent system was found to be correlated with the Kamlet-Taft solvent parameters. Moreover, the use of microwave heating also enhances the efficiency of lignocellulose pretreatment. For example, the microwave-assisted [Emim][OAc]-DMSO (1:1 volume ratio) treated-rice straw could be hydrolyzed at least 22 times faster than that of untreated-rice straw by cellulase from Trichoderma reesei. This enhancement was attributed by several factors including more efficient lignin extraction, less crystalline cellulose and lower residual ILs in treated-rice straw. The produced sugars can be effectively fermented by Pichia stipitis for ethanol production. Moreover, [Emim][OAc]-DMSO mixture could be reused at least 5 times without significantly decrease in effectiveness demonstrated that the use of ILs/co-solvent was potential alternative method for large-scale biomass pretreatment. (C) 2014 Elsevier Ltd. All rights reserved.