A novel pretreatment method-concerted ionic liquid (CIL) pretreatment was proposed and the mechanism of CIL pretreatment to overcome the recalcitrance of biomass was investigated in this study. The CIL refers to a combination of a protic ionic liquid (PIL) pyrrolidinium acetate ([Pyrr][AC]) and an aprotic ionic liquid (AIL) 1-butyl-3-methylimidazolium acetate ([BMIM][AC]) in varied proportion. In the CIL system, [Pyrr][AC] selectively dissolves lignin from biomass and [BMIM][AC] dissolves part of cellulose and hemicelluloses. Besides lignin, hemicelluloses are also an important factor of biomass recalcitrance. Optimized [BMIM][AC] fraction in the CIL pretreatment resulted in the efficient removal of lignocellulose complex while [Pyrr][AC] selectively fractionated lignin component. CILs exhibited superior efficiency in overcoming the recalcitrance of eucalyptus bark over that of the individual constituent ionic liquids. Compared to pretreatments by individual ILs of the pair, the CIL pretreatment increased the glucose yield by 30% from the cellulose of eucalyptus bark. High cellulose conversion of 91% (wt%) was achieved from enzymatic conversion of eucalyptus bark by optimizing the lignin and hemicelluloses removal. (C) 2015 Elsevier Ltd. All rights reserved.