Pretreatment of switchgrass using aqueous ammonia or NaOH was investigated as a means to enhance the enzymatic digestibility. To increase the effectiveness of pretreatment, H(2)O(2) was supplemented with the alkaline reagents. Since H(2)O(2) is unstable at high temperature, low-to-high step-change of temperature was applied, and this scheme was found to be effective in the case of NH(3)/H(2)O(2) treatment. The composition of pretreatment liquid indicates that hemicellulose solubilized during alkaline treatment exists either in the form of oligomers or as lignin-carbohydrate complex (LCC). LCC formation was prominent in ammonia treatment and in NaOH/H(2)O(2) treatment, and that LCC formation protects the hemicelluloses sugars from degradation. Lignin in the pretreatment liquor was precipitated and subjected to TGA and FTIR analyses. TGA data indicate that NaOH-lignin has more uniform structure and higher O/C ratio than ammonia-lignin. FTIR analysis indicates that NaOH-lignin has lower aromatic content but higher guaiacyl type structure than ammonia-lignin. (c) 2010 Elsevier Ltd. All rights reserved.