Abstract: Prior to cooking, fragments of raw ligno-cellulosic material are impregnated with a solution of a lignin-solubilizing reactant in an organic solvent which has a boiling point higher than the cooking temperature in order for the material to absorb an amount of solution containing at least the amount of reactant required for solubilizing the lignin, then the impregnated material is immersed in a liquid which is substantially non-miscible with the solvent of said solution, said liquid having a boiling point higher than the cooking temperature, and said liquid is maintained at the cooking temperature until the lignin has been substantially converted to a derivative which is soluble in water or a water-miscible organic solvent.