Ionic liquids (ILs) have shown great potential for the reduction of lignin in biomass after pretreatment. Although dilute acid and base pretreatments have been shown to result in pretreated biomass with substantially different lignin composition, there is scarce information on the composition of lignin of IL pretreated biomass. In this work, temperature dependent compositional changes in lignin after IL pretreatment were studied to develop a mechanistic understanding of the process. Panicum virgatum and Eucalyptus globulus were pretreated with 1-ethyl-3-methylimidazolium acetate ([C(2)mim][OAc]). Measurement of syringyl and guaiacyl ratio using pyrolysis-GC/MS and Kamlet-Taft properties of [C(2)mim][OAc] at 120 degrees C and 160 degrees C strongly suggest two different modes of IL pretreatment. Preferential breakdown of S-lignin in both eucalyptus and switchgrass at high pretreatment temperature (160 degrees C) and breakdown of G-lignin for eucalyptus and no preferential break down of either S- or G-lignin in switchgrass was observed at lower pretreatment temperatures (120 degrees C). (C) 2012 Published by Elsevier Ltd.