In this study, 4-methylmorpholine N-oxide (NMMO) and four ionic liquids were compared to determine the most suitable type for use in pretreatment of oil palm empty fruit bunch (EFB) to enhance enzymatic digestibility and obtain maximal fermentable sugar (glucose). Four types of ionic liquids, namely, 1-ethyl-3-methyl imidazolium acetate ([Emim]OAc), 1-butyl-3-methyl imidazolium chloride ([Bmim]Cl), tributyl methyl ammonium methylsulfate (MTBS), and 1-ethyl-3-methyl imidazolium diethylphosphate ([Emim]DEP), were used. The effects of pretreatment temperature and time on the enzymatic digestibility of EFB were examined. Among NMMO and four ionic liquids, NMMO and [Emim]OAc, showing high enzymatic digestibility, were found to be most effective for EFB pretreatment. The enzymatic digestibility of NMMO-pretreated EFB increased as pretreatment temperature and time increased. After [Emim]OAc pretreatment, the crystallinity index of EFB decreased, due to the conversion of crystalline cellulose to amorphous cellulose, and the enzymatic digestibility thereby was increased to 96.6% at 110 degrees C for 2 h. (C) 2012 Elsevier Ltd. All rights reserved.