Chitosans were reported to possess antioxidant effects both in vitro and in vivo [56-58]. Low-molecular-weight chitosans and their derivatives were more pronounced in antioxidant capacity than high-molecular-weight ones . Tomida et al.  have detailed that low-molecular-weight chitosans were more effective in preventing the formation of carbonyl groups in plasma protein exposed to peroxyl radicals and also found to effectively prevent the formation of carbonyl groups in human serum albumin when exposed to peroxyl radicals. Moreover, they were also good scavengers of N-centred radicals, but high-molecular-weight chitosans were much less effective. Chitosans have two hydroxyl groups and one amino group in each of their monosaccharide construction units, and the hydroxyl groups in the polysaccharide units can react with free radicals by the typical H-abstraction reaction . In addition, according to free radical theory, the amino groups in chitosan can react with free radicals to form additional stable macroradicals. Therefore, the active hydroxyl and amino groups in the polymer chains are the origin of the scavenging ability of chitosan. High-molecular-weight chitosans have a compact structure, and the effect of intramolecular hydrogen bond is stronger. The strong effect of the intramolecular hydrogen bond weakens the activities of the hydroxyl and amino groups, and the chance of exposure of their hydroxyl and amine groups might be restricted, which would account for less radical-scavenging activities . Antioxidant activities of various chitosan derivatives were also reported. Modification of chitosan by adding quaternium on amino groups found to help increase the antioxidant activity . Ying et al.  prepared various water-soluble Schiff base typed chitosan saccharide derivatives and investigated for their antioxidant activities, and these water-soluble chitosan derivatives exhibited higher ability of scavenging 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical compared with chitosan. The superoxide anion scavenging activity of CMCS was studied by Sun et al. , and the results demonstrated that the scavenging rate increases with the increase in concentration. At the maximum concentration of about 1200 mg mL−1, it has scavenging rate of 13.4%. Moreover, antioxidant activities of N-carboxymethyl chitosan oligosaccharides with different degrees of substitution (DS 0.28, 0.41, and 0.54) were evaluated by the scavenging of DPPH radical, superoxide anion and the determination of reducing power. With the increase in substituting degree, the scavenging activity of N-CMCS against DPPH radical decreased and reducing power increased. As for superoxide anion scavenging, the N-CMCS of 0.41 DS is higher in activity than those of 0.54 and 0.28 DS. The unrelated results obtained may interrelate to the different radical-scavenging mechanisms and donating effect of substituting carboxymethyl group . In another study by Yang et al. , the chitosan, hyaluronan and starch-carboxymethylated derivatives showed lower scavenging ability on hydroxyl radicals than the parent compounds. Decrease in scavenging ability of the carboxymethylated polysaccharides could be rationalized by the fact that part of the hydroxyl groups was substituted by carboxyl groups in the molecules. For the three kinds of polysaccharides, scavenging ability on hydroxyl radicals was found to be in the order of chitosan > hyaluronan > starch. The scavenging ability of carboxymethylated polysaccharides had the same order as related to its corresponding polysaccharides at higher concentrations (≥0.8 mg mL−1). There were not only hydroxyl groups but also amino or acetamino (CH3CONH–) groups in the molecules of chitosan and hyaluronan, but only hydroxyl group for starch. Thus, it was suggested that the influence of the scavenging activity against hydroxyl radicals might be in the sequence of amino group > acetamide group > hydroxyl group. However, in another work reported by Zhao et al. , N,O-CMCS from squid cartilage showed a better antioxidant than chitosan, especially in terms of its reducing power, scavenging ability towards DPPH and superoxide radicals, and chelating ability of ferrous ions. The data on the antioxidant capacities obtained may be due to the type of β-chitin in squid cartilage, which possessed weak intermolecular forces and exhibited higher reactivity under various modification conditions as well as higher affinity for solvents than α-chitin. The introduction of hydrophilic carboxymethyl groups on squid cartilage chitosan decreased the intramolecular and intermolecular hydrogen bonds, resulting in the exposure of more hydroxyl groups, thus the scavenging abilities of N,O-CMCS are better than that of chitosan. It is worth mentioning at this point again that the antioxidant activities of antioxidant compounds are attributed to various mechanisms, among which are prevention of chain initiation, binding of transition metal ion catalysts, decomposition of peroxides, reductive capacity and radical scavenging. The data on activities of chitosan and carboxymethyl chitosan suggested that these various mechanisms likely to significantly contribute towards the observed antioxidant effects. Oligochitosan and N,O-CMCS also have been reported on inhibiting 50% of ABTS radical formation. Oligochitosans displayed IC50 values of 3.5 to 4.6 mg mL−1, whereas N,O-CMCS is a more effective free radical scavenger as it possessed the IC50 value of 0.98 ± 0.07 mg mL−1 . On the basis of the data obtained from all the mentioned works, it may be reasonable to claim that the antioxidant activities of water-soluble chitosan derivatives are medium and would be rather useful option to expand their applications in foods, pharmaceuticals and cosmetics [59, 64, 67, 69].