NOVELTY - Fractionating biomass involves: (a) providing biomass feedstock comprising cellulose, hemicellulose and lignin; (b) fractionating feedstock in digestor in presence of acid catalyst, solvent for lignin, and water to produce a liquor; (c) removing cellulose-rich solids from liquor; (d) hydrolyzing hemicellulose, to produce hemicellulosic monomers; (e) optionally hydrolyzing cellulose-rich solids to produce glucose; and (f) recovering the monomers, and optionally glucose if step (e) is conducted, to produce fermentable sugars, where additive is introduced in step (b) and/or (d).    USE - For fractionating biomass (claimed) useful for converting biomass into fermentable sugars, cellulose, and lignin which are useful as intermediates for chemical and fuel production.    ADVANTAGE - The method produces cellulose-rich solids while concurrently producing a liquid phase containing a high yield of both hemicellulose sugars and lignin, and low quantities of lignin and hemicellulose degradation products. The flexible fractionation technique enables multiple uses for the products. The sulfite/bisulfite additives produce lignosulfonates and prevent lignin from extensive condensation; and effectively depolymerize lignin, to some extent, thereby reversing acid-catalyzed condensation that may have taken place. The presence of the additive reduces precipitation of the lignin in the digestor or in secondary hydrolysis. The sulfonated lignin is hydrophilic and may have reduced tendency to agglomerate, compared to the starting lignin. When the solvent is water-miscible, a single liquid phase forms, so mass transfer of lignin and hemicellulose extraction is enhanced, and the downstream process must only deal with one liquid stream. When the solvent is immiscible in water, the extractant mixture readily separates to form liquid phases, so a distinct separation step can be avoided or simplified. This can be advantageous if one liquid phase contains most of the lignin and the other contains most of the hemicellulose sugars, as this facilitates recovering the lignin from the hemicellulose sugars.    DETAILED DESCRIPTION - Fractionating biomass involves: (a) providing a biomass feedstock comprising cellulose, hemicellulose, and lignin; (b) in a digestor, fractionating feedstock under effective fractionation conditions in presence of an acid catalyst, solvent for lignin, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; (c) removing cellulose-rich solids from liquor; (d) hydrolyzing hemicellulose contained in liquor, under effective hydrolysis conditions, to produce hemicellulosic monomers; (e) optionally hydrolyzing cellulose-rich solids to produce glucose; and (f) recovering hemicellulosic monomers, and optionally glucose if step (e) is conducted, to produce fermentable sugars, where an additive is introduced in step (b) and/or step (d), and at least a portion of additive reacts directly or indirectly with lignin to produce sulfonated lignin. The additive is selected from sulfite anions, sulfite salts, bisulfite anions, and/or bisulfite salts.