Embodiments of the present invention overcome the well-known recalcitrance of lignocellulosic biomass in an economically viable manner. A process and system are provided for the efficient fractionation of lignocellulosic biomass into cellulose, hemicellulose sugars, lignin, and acetic acid. The cellulose thus obtained is highly amorphous and can be readily converted into glucose using known methods. Fermentable hemicellulose sugars, low-molecular-weight lignin, and purified acetic acid are also major products of the process and system. The modest process conditions and low solvent/solid ratios of some embodiments of the invention imply relatively low capital and processing costs.