Chitosan derivatives having structural units of formula I, II and III in random distribution ##STR1## wherein the substituents R.sub.1 are each, independently of one another, H or --Z--R.sub.2 --X, R.sub.3 is H or acetyl, Y is the anion O--Z--R.sub.2 --X, and (a) Z is --CO-- or --SO.sub.2 --, X is --CO.sub.2 H, --CH.sub.2 CO.sub.2 H or --CH.sub.2 PO(OH).sub.2, and R.sub.2 is --CHR.sub.4 CR.sub.5 (OH)--, R.sub.4 is --H, --OH, C.sub.1 -C.sub.4 alkoxy or C.sub.1 -C.sub.4 alkyl, and R.sub.5 is H or C.sub.1 -C.sub.4 alkyl, or (b) Z is --CO--, X is --CO.sub.2 H, and R.sub.2 is --CHR.sub.6 --CHR.sub.7 --CH(OH)-- or --CHR.sub.8 --CHR.sub.9 --CHR.sub.10 --CH(OH)--, R.sub.6, R.sub.7, R.sub.8 and R.sub.10 are each, independently of one another, --H, --OH, C.sub.1 -C.sub.4 alkyl or C.sub.1 -C.sub.4 alkoxy, and R.sub.9 is --H, --OH, C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy or --CO.sub.2 H, and the esters and salts thereof, which chitosan derivative contains a total of at least 2 structural units and, based on 1 mol of the chitosan derivative, 30 to 100% molar of structural units of formula I, 60 to 0% molar of structural units of formula II, and 30 to 0% molar of structural units of formula III, and the sum of the molar percentages is 100%. These derivatives are suitable for use as humectants and for the prevention of the adherence to and/or formation of solid deposits on inorganic or organic substrates.