TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The process is carried out at 160-230 degrees C, preferably 180-210 degrees C. The biomass is submitted to a preliminary milling process before being put in contact with the aqueous solution of at least one organic acid. The biomass is milled until obtaining particles having a diameter of 0.1-10 mm. The biomass is put in contact with the aqueous solution for 1-8 (preferably 2-5) hours. The process comprises obtaining a first solid phase and a first aqueous phase. The organic acid is the organic acid that is put in contact with the biomass. The organic water-insoluble solvent is evaporated obtaining a second solid phase comprising the organic acid and a second aqueous phase comprising the 5-6C sugar. The process comprises reusing the organic acid. The second aqueous phase comprising at least one 5-6C sugar is used directly in fermentation processes for alcohol production. The first aqueous phase is submitted to extraction with an organic water-insoluble solvent. Preferred Components: The organic acid is alkylsulfonic acids, arylsulfonic acids or halogenated carboxylic acids. The alkylsulfonic acids have a structure of formula (R-SO3H). The arylsulfonic acids have a structure of formula (Ar1 (SO3H)n). The halogenated carboxylic acids have a structure of formula (Rx-COOH). The acid is para-toluenesulfonic acid, 2-naphthalenesulfonic acid, 1,5-naphthalenedisulfonic acid (all preferred), dodecylsulfonic acid, 1-naphthalenesulfonic acid, 2,5-naphthalenedisulfonic acid, trifluoroacetic acid, dichloroacetic acid, trichloroacetic acid, and/or perfluorooctanoic acid. The polysaccharide is cellulose and/or hemicellulose. The biomass is a lignocellulosic biomass. The biomass is present at 5-40 (preferably 10-20) wt.%. The organic acid is water-soluble and extractable with an organic water insoluble solvent. The organic acid is present at 0.1-5 (preferably 0.5-2) wt.%. The first solid phase comprises lignin. The first aqueous phase comprises at least one 5-6C sugar and the organic acid. The organic water-insoluble solvent is halogenated hydrocarbons, aromatic hydrocarbons, and/or 4-6C aliphatic alcohols. The organic water-insoluble solvent is toluene and a mixture of toluene/n-butanol.R = 4-16C cyclic alkyl;Ar1 = 6-12C aryl;Rx = 1-10C cyclic alkyl containing at least one halo substituent; andn = 1-2.Preferred Components: The lignocellulosic biomass is: the products of cultivations expressly cultivated for energy use, comprising refuse, residues and waste of the cultivations or their processing; the products of farm cultivations, forestation and sylviculture, comprising wood, plants, residues and refuse of farm processings, forestation and sylviculture; the refuse of agri-food products intended for human feeding or zootechnics; the non-chemically treated residues of the paper industry; and the waste from selective solid urban waste collection. EXAMPLE - Milled coniferous wood flour (10 g) was added to a solution of 2-naphthalenesulfonic acid (1 g) in water (100 ml). The mixture was kept under stirring in an autoclave, at 200 degrees C for 4 hours. After cooling, the solid phase was separated by filtration and dried obtaining lignin (2.5 g) with a purity of greater than 95%. The aqueous solution was extracted with 300 ml of a mixture of toluene/n-butanol (3:1 by volume). The organic phase was then separated and the solvent was evaporated at reduced pressure, obtaining solid residue consisting of 2-naphthalenesulfonic acid (0.97 g). After extraction, the aqueous phase has a pH equal to 6 and contains a glucose with a concentration of 4.2%.