Biomass pretreatment is one of major bottlenecks to convert biomass to bioethanol at present. We found that cereal straw could be completely dissolved in a 10-fold volume excess of trifluoroacetic acid (>= 99%) (TFA). Pretreatment with TFA completely disrupted the dense cellulose crystallinity of the biomass. Further research showed that the TFA dissolved, but did not degrade, the cellulose in the straw. TFA did, however, degrade 65.65% of xylan in the straw, as well as reduce 20.0-23.3% of the acid-insoluble lignin. Isopropanol could precipitate 92.7% of the material dissolved in the TFA solution. These results led us to design a procedure for pretreatment of cellulosic biomass involving a dissolving step with TFA and a precipitation step with isopropanol. Experiments show that the procedure is technically feasible. Moreover, TFA and isopropanol could be completely evaporated off the supernatant and precipitate, and recycled back into the process. (C) 2009 Elsevier Ltd. All rights reserved.