A biomass pretreatment process was developed using acidified ionic liquid (IL) solutions containing 10-30% water. Pretreatment of sugarcane bagasse at 130 degrees C for 30 min by aqueous 1-butyl-3-methylimidazolium chloride (BMIMCl) solution containing 1.2% HCl resulted in a glucan digestibility of 94-100% after 72 h of enzymatic hydrolysis. HCl was found to be a more effective catalyst than H2SO4 or FeCl3. Increasing acid concentration (from 0.4% to 1.2%) and reaction temperature (from 90 to 130 degrees C) increased glucan digestibility. The glucan digestibility of solid residue obtained with the acidified BMIMCl solution that was re-used for three times was >97%. The addition of water to ILs for pretreatment could significantly reduce IL solvent costs and allow for increased biomass loadings, making the pretreatment by ILs a more economic proposition. (C) 2012 Elsevier Ltd. All rights reserved.