TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Process: The pre-hydrolysis is conducted in mild conditions for acid concentration and temperature and separate partial or totally the sugar solution (monomers and oligomers) before feeding the residual cellulosic material for the cellulose hydrolysis step. The cellulosic hydrolysis step is conducted in more severe conditions for acid concentration and temperature and the sugar solution (monomers and oligomers) is removed as an isolated chain or mixed with sugar chain driving from the pre-hydrolysis step. The lignin solution and the sugar solutions are obtained in separated currents. Optionally the sugar current rich in pentose monomers and oligomers is obtained separately from the sugar current rich in hexose monomers e oligomers. The effluent solid material currents of the first and second steps are washed out with water or optionally with a diluted sugar solution. The lignin dissolution step is made using a solvent from ketones such as acetone, methyl ethyl ketone and/or methyl isobutyl ketone, in neutral or slightly acid media; aliphatic alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-l-butanol, 1pentanol, 2-methyl-l-butanol, 3-methyl-l-butanol or ethyl, propyl and/or butyl glycols, in neutral, alkaline or slightly acid media; mixtures of ketone, aliphatic alcohols and/or glycols in neutral or slightly acid media and its mixtures in neutral, alkaline or slightly acid media; mixtures of ketones and aliphatic alcohols in neutral or these solvents being either anhydrous or, preferentially, in slightly acid media, with aqueous solutions. The temperature in the lignin dissolution step is 70-200degreesC. The major part of the lignin solution is separated from the residual solids soon after the formation of the referred solution. The pre-hydrolysis step is conducted to obtain a yield in sugars over 85% than theoretically could be obtained from the hydrolyzed cellulosic material, being able to reach 95%, if the conditions were optimized. The pre-hydrolysis step is conducted in current parallel with an acid solution, being that acids such as sulfuric, nitric, phosphoric and hydrochloric can be used. The temperature in the pre-hydrolysis step is 70-200 (preferably 100-160)degreesC. The heating can be made optionally by the liquid recirculation, impelling by a pump, through an external heater of the reactor, and where the heated liquid returns to the reactor. The heating can be made optionally by live steam injection. The washing water or, optionally a diluted sugar solution, is fed to an elevated temperature in such a way to reduce or eliminate the need of heat supply through recirculation through external heater or by the live steam introduction.Preferred Composition: The sugar solution that leaves the hemicellulose hydrolysis step is 5-35 wt.%. The acid concentrations can vary at 0.1-2 g/L, for a residence time of 1-20 minutes.