Effects of 360-kHz ultrasound on aqueous solutions of chitosan and starch were studied. This treatment is an efficient procedure for reduction of molecular weight of both polysaccharides. It has been demonstrated that at the applied ultrasound frequency degradation is caused both by OH radicals and mechanochemical effects. In Ar-saturated 2×10?2 mol dm?3 chitosan solutions, pH 3, at an ultrasound dose rate of 170 W kg?1, the average sonochemical chain scission yield in the sonication time range of 090 min is ca. 8×10?11 mol J?1. This yield has been shown to depend on polymer concentration, ultrasound power and gas used to saturate the solution. Scission is accompanied by side reactions leading to the formation of carbonyl groups. Ultrasound-induced chain scission of starch proceeds with lower yield, one of the reasons being probably the different chain conformation when compared with rod-like chitosan macromolecules.