Cellulose is produced from ligno-cellulosic material, such as woods and straws, by heating the material with a liquid comprising an unsaturated dicarboxylic acid or a cyclic anhydride of such an acid. It is preferred to use maleic or fumaric acid or maleic anhydride, but chloromaleic acid and its anhydride, itaconic acid and its anhydride, citraconic acid, dimethyl maleic anhydride or methyl ethyl maleic anhydride may also be used. The unsaturated dicarboxylic acid or anhydride may be employed alone (provided that its melting point is below the treating temperature) but is preferably used in admixture with a solvent such as water, an alcohol, e.g. ethyl, isopropyl, butyl or amyl alcohol, a saturated lower fatty acid, e.g. acetic, propionic or butyric acid, or (when an unsaturated anhydride is employed) a hydrocarbon, especially an aromatic hydrocarbon such as benzene, toluene or the xylenes. Particularly useful mixtures are maleic or fumaric acid with water or acetic acid and mixtures of maleic anhydride with acetic acid or with one or more aromatic hydrocarbons. The proportion of treating agent used is preferably at least twice, and the proportion of agent plus solvent (if used) about 6-20 times the weight of the ligno-cellulosic material. The concentration of the treating agent in the treating liquid is preferably at least 10 per cent. by weight. The treating liquid may contain a small proportion, e.g. about 2 per cent. or less based on the treating agent, of a mineral acid, especially hydrochloric acid, or of an organic peroxide, e.g. benzoyl peroxide, which functions as a catalyst. The treating temperature is usually above 80 DEG C. and especially 100-190 DEG C. and the treating liquid preferably boils under atmospheric pressure at or above the treating temperature. At temperatures above about 110 DEG C. the treating time is usually 1-5 hours. After the treatment the impure cellulose obtained may be washed with further quantities of solvent, and further purified by heating to 70-100 DEG C. with an aqueous 2-20 per cent., and especially 3-6 per cent., caustic soda solution. The alkali treatment may be repeated two, three or more times. The lignin dissolved in the treating liquid may be recovered therefrom (see Group IV (b)), or the treating agent may be removed from the treated material by vacuum distillation and the treated material freed from lignin by treating with 7-12 per cent. aqueous caustic soda solution at about 70-100 DEG C. In an example, 25 parts by weight of poplar chips are heated to 120 DEG C. for 2.5 hours with a mixture of 100 parts of maleic anhydride and 100 parts of acetic acid. The acid liquor is then run off and the chips are washed successively with acetic acid water, three times with hot 4 per cent. aqueous caustic soda solution and finally with hot water. In another example 50 parts of poplar chips are similarly treated with a mixture of 250 parts of maleic anhydride, 250 of toluene and 20 of benzoyl peroxide. The treated material after alkali treatment may be used without bleaching or other supplementary purification. Specification 686,311 is referred to.ALSO:Lignin is obtained from ligno-cellulosic material, such as woods and straws, by heating the material with a liquid comprising an unsaturated dicarboxylic acid or a cyclic anhydride of such an acid, preferably maleic or fumaric acid or maleic anhydride (see Group IV (a)). The treating agent may be used alone but is preferably used in admixture with a solvent for the agent such as water, an alcohol, e.g. ethyl, isopropyl, butyl or amyl alcohol, a saturated lower fatty acid, e.g. acetic, propionic or butyric acid, and (when the agent is an anhydride), a hydrocarbon, especially an aromatic hydrocarbon such as benzene, toluene or the xylenes. A small proportion of a mineral acid, e.g. HCl, or of an organic peroxide, e.g. benzoyl peroxide, may be present. The extraction temperature is usually above 80 DEG C. and especially 100-190 DEG C. The extract of lignin is separated from the delignified cellulose and the lignin may be precipitated by neutralizing or diluting the extract. For example, when the extraction liquid is a mixture of maleic or fumaric acid with acetic acid, the acetic acid may be removed by distillation and the residue diluted with water; when the treating liquid comprises a mixture of maleic anhydride and toluene or other aromatic hydrocarbon, the lignin may be precipitated by further diluting the anhydride with an aromatic hydrocarbon such as benzene. Alternatively the lignin may be precipitated by distilling off the unsaturated acid or anhydride, preferably under reduced pressure. In an example, poplar chips are heated with a mixture of maleic acid and acetic acid, the acid liquor is run off, the treated chips are washed with acetic acid and the combined washings and maleic acid liquor heated to distil off the acetic acid and diluted with water to precipitate the lignin. Specification 686,311 is referred to.