The present invention provides a process for fractionating lignocellulosic biomass, comprising: contacting biomass with SO2, water, and optionally a first solvent, to produce intermediate solids; then contacting the intermediate solids with SO2, water, and a second solvent, to produce cellulose-rich solids and a liquid phase comprising hemicelluloses and lignin. The first concentration of SO2 may be lower or higher than the second concentration of SO2. It is desirable to vary the SO2 and solvent concentrations in different stages to optimize the removal of hemicellulose versus lignin. The resulting cellulose-rich material can contain very low hemicellulose, very low lignin, or both low hemicellulose and low lignin. High-purity cellulose is useful both for producing glucose as well as for cellulose products or derivatives. The hemicelluloses may be hydrolyzed to produce monomeric sugars, and the lignin may be recovered as a co-product.
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