NOVELTY - Fractionating lignocellulosic biomass into cellulose rich solid fraction and liquid fraction, involves adding alkaline agent is selected from alkali metal carbonates, hydroxides and percarbonates and water to the biomass to form mixture having alkaline pH; contacting the biomass in the mixture with oxidizing agent at elevated temperature; and continuing the contacting of the biomass with oxidizing agent until most of the lignin is solubilized to form solid fraction containing cellulosic material having reduced content of lignin and liquid fraction containing dissolved lignin.    USE - For fractionating lignocellulosic biomass into cellulose rich solid fraction and liquid fraction, where the biomass is selected from wood, softwood, pulp, recycled fibers, straw, sugar cane bagasse, agricultural, municipal and industrial wastes and similar compositions which contain carbohydrates (claimed); and for the production of chemicals and for other non-fibrous applications.    ADVANTAGE - The present method is efficiently modifying the lignocellulosic material with greatly reduced formation of inhibiting compounds and without using a catalyst by lessening the problems related to current pre-treatment techniques. The carbohydrates obtained with the present method are more susceptible to enzymatic hydrolysis making the further processing of plant material more feasible than with known methods. The present method provides efficient conversion of any lignocellulosic material (wood, straw, and bagasse) to hydrolysable form; and provides separation of lignin from lignocellulosic materials and dissolution of the lignin thus separated into the water phase.    DETAILED DESCRIPTION - Fractionating lignocellulosic biomass into at least one cellulose rich solid fraction and at least one liquid fraction, involves adding alkaline agent and water to the biomass to form mixture having alkaline pH, the alkaline agent is selected from alkali metal carbonates, hydroxides and percarbonates; contacting the biomass in the mixture with oxidizing agent at elevated temperature; and continuing the contacting of the biomass with the oxidizing agent until most of the lignin is solubilized to produce solid fraction comprising cellulosic material having reduced content of lignin and liquid fraction comprising dissolved lignin.