By the reaction of chitosan with iminothiolactones such as 2-iminothiolane, thiol groups can be covalently immobilized on the polymer. The resulting chitosan derivatives exhibit thiol groups which are capable of forming inter- and intramolecular disulfide bonds. Because of this crosslinking process the viscosity of polymer solutions is strongly improved. Moreover, the cationic character of the polymer is strongly improved. Preferred applications of these novel chitosan derivatives are their cosmetic and pharmaceutical use ##STR00001##