The invention relates to a method for chemically modifying polysaccharides with the aid of a mechanical device and of at least one modifying reagent. The method is characterized in that the polysaccharide constituent is subjected at least once to a treatment by a roll mill during which at least two adjacent and counter-rotating rolls rotate at different speeds, and the polysaccharide constituent is mixed with the modifying reagent before and/or during the mechanical treatment. During this method, typically pectins, carob seed grain, guar meal and alginates are used as the polysaccharide constituent, and epoxides, amines or carboxylic acid derivatives are used as modifying reagents. The mechanical treatment can be repeated one to three times in a multiple roll mill, preferred rotating speeds of the adjacent rolls typically differing by 200%. The polysaccharides, which are mechanically modified in an extremely homogeneous manner according to the inventive method, are preferably used as thickening agents, gelling agents, emulsifiers, food additives, cosmetic additives, as well as hair and fiber care agents.