The switchgrass (SG) samples pretreated by cellulose solvent- and organic solvent-based lignocellulose fractionation were characterized by enzymatic hydrolysis, substrate accessibility assay, scanning electron microscopy, X-ray diffraction (XRD), cross polarization/magic angle spinning (CP/MAS) C-13 nuclear magnetic resonance (NMR), and Fourier transform infrared spectroscopy (FTIR). Glucan digestibility of the pretreated SG was 89% at hour 36 at one filter paper unit of cellulase per gram of glucan. Crystallinity index (CrI) of pure cellulosic materials and SG before and after cellulose solvent-based pretreatment were determined by XRD and NMR. CrI values varied greatly depending on measurement techniques, calculation approaches, and sample drying conditions, suggesting that the effects of CrI data obtained from dried samples on enzymatic hydrolysis of hydrated cellulosic materials should be interpreted with caution. Fast hydrolysis rates and high glucan digestibilities for pretreated SG were mainly attributed to a 16.3-fold increase in cellulose accessibility to cellulase from 0.49 to 8.0 m(2)/g biomass, because the highly ordered hydrogen-bonding networks in cellulose fibers of biomass were broken through cellulose dissolution in a cellulose solvent, as evidenced by CP/MAS C-13-NMR and FTIR. Biotechnol. Bioeng. 2011; 108: 521-529. (C) 2010 Wiley Periodicals, Inc.