A new water-soluble derivative of chitosan — chitosan oligoethylene oxide sulfonate — was synthesized. The ratio and concentration of reacting substances and temperature and duration of the chitosan modification reaction affect the composition and properties of the reaction products. The conditions that ensure a 100% yield of water-soluble chitosan derivatives were determined. Using potentiometric titration and IR spectroscopy, the formation of a new water-soluble chitosan derivative — oligoethylene oxide sulfonate — was confirmed.