Author Proof future science group 33 ISSN 1759-7269 10.4155/BFS.09.5 © 2010 Future Science Ltd Increasing greenhouse gas emissions, rising oil prices and decreasing natural petroleum-based resources under- score the need for low-carbon, renewable transportation fuels [1–6] . Today, ethanol is perceived as the renewable biofuel of choice within the global transportation sector for the near-term. Ethanol is currently produced in large quantities from cornstarch or sugarcane in Brazil, the USA and China. However, producing ethanol from these edible feedstocks will not be able to meet the produc- tion targets for biofuel production [3,4,6] . Lignocellulosic biomass, such as agricultural and forest residues and dedicated energy crops, is recognized as one of the few significant renewable resources available worldwide and can be utilized for the production of sustainable biofuels at competitive prices [1,3,6–8] . It is estimated that 30% of the petroleum consumed currently by the USA could be replaced by lignocellulosic-based renewable biofuel [4] . Obtaining biofuels from lignocellulosic biomass involves three major steps [5,6,9] :  Pretreatment of biomass for enhanced accessibility and digestibility by enzymes;  Hydrolysis of cellulose and hemicelluloses to fermentable reducing sugars;  Fermentation of sugars to designated biofuels. Biofuels (2010) 1(1), 33–46 Understanding the impact of ionic liquid pretreatment on eucalyptus Özgül Persil Çetinkol 1,2 , Dean C Dibble 1,3 , Gang Cheng 1,3 , Michael S Kent 1,3 , Bernhard Knierim 2,4 , Manfred Auer 1,2,4 , David E Wemmer 5 , Jefrey G Pelton 5 , Yuri B Melnichenko 6 , John Ralph 7 , Blake A Simmons 1,3 & Bradley M Holmes 1,3† Background: The development of cost-competitive biofuels necessitates the realization of advanced biomass pretreatment technologies. Ionic liquids provide a basis for one of the most promising pretreatment technologies and are known to allow efective processing of cellulose and some biomass species. Results and discussion: Here, we demonstrate that the ionic liquid 1-ethyl-3-methyl imidazolium acetate, [C2mim] [OAc], induces structural changes at the molecular level in the cell wall of Eucalyptus globulus. Deacetylation of xylan, acetylation of the lignin units, selective removal of guaiacyl units (increasing the syringyl:guaiacyl ratio) and decreased b-ether content were the most prominent changes observed. Scanning electron mi- croscopy images of the plant cell wall sections reveal extensive swelling during [C2mim][OAc] pretreatment. X-ray difraction measurements indicate a change in cellulose crystal structure from cellulose I to cellulose II after [C2mim][OAc] pretreatment. Enzymatic sacchariication of the pretreated material produced increased sugar yields and improved hydrolysis kinetics after [C2mim][OAc] pretreatment. Conclusion: These results provide new insight into the mechanism of ionic liquid pretreatment and reairm that this approach may be promising for the production of cellulosic biofuels from woody biomass. † Author for correspondence; E-mail: bmholme@sandia.gov 1 Joint BioEnergy Institute, Deconstruction Division, Emeryville, CA, USA 2 Lawrence Berkeley National Laboratory, Physical Biosciences Division, Berkeley, CA, USA 3 Sandia National Laboratories, Livermore, CA, USA, and Albuquerque, NM, USA 4 Joint BioEnergy Institute, Technology Division, Emeryville, CA, USA 5 Department of Chemistry and California Institute for Quantitative Biosciences, University of California, Berkeley, CA, USA 6 Oak Ridge National Laboratory, Spallation Neutron Source, Knoxville, TN, USA 7 DOE Great Lakes BioEnergy Research Center, and Departments of Biochemistry and Biological Systems Engineering, University of Wisconsin-Madison, Madison, WI, USA RESEARCH ARTICLE