The chemical synthesis of polysaccharides is an old problem, wh ich has been studied for several decades without noteworthy success. We have accomplished this synthesis in three different ways. The most comprehen- sive in application is the synthesis from mono-or disaccharides in dimethyl sulphoxide as solvent, at room temperature and under H-ion catalysis. If the water formed in the equilibrium between mono- and polysaccharides is removed, polysaccharides of molecular weights up to about 30. 000 are obtained. These are still soluble in water. Substantially higher molecular weight polysaccharides are not water- soluble. Such polysaccharides were obtained from o-glucose, o-galactose, o-mannose, o-xylose, maltose, cellobiose, and lactose. The structural deter- mination showed that they have, in the main, a highly branched structure. Since polysaccharides containing amino-sugars and uronic-acids occur very widely in nature, such polymers were synthesised in a similar manner from o-glucosamine, and others which contained o-glucuronic acid and o-glucose, o-glucuronic acid and o-glucosamine and o-glucose and o-glucosamine. Poly- saccharides could be obtained by another method, from 1-fluoro-sugars in acid medium. The mechanism of this synthesis is different. Polysaccharides are formed from both hexose and pentose derivatives. A synthesis from 1-fluor-sugars in alkaline medium was more stereospecific. It is only suc- cessful if for steric reasons, the formation of products of low molecular weight is not possible.