Process for treating keratin materials, in particular keratin fibres, especially human keratin fibres, preferably the hair, comprising at least one step consisting in applying, to said fibres, a) one or more oxidized amino polysaccharide(s) comprising at least one carboxy group -C -OH or carboxylate group -C O -, M + with M + representing a cationic counterion, in which said oxidized polysaccharide(s) a) are represented by formula below: (R 1 R 2 N) p -P- m (COOX) n in which formula: • P represents a polysaccharide chain constituted of monosaccharides comprising 5 carbon atoms or more than 5 carbon atoms, preferably 6 or more than 6 carbon atoms and more particularly 6 carbon atoms; • R 1 and R 2 represent a hydrogen atom; • X represents a hydrogen atom, a cationic counterion, in particular chosen from i) alkali metal, ii) alkaline earth metal, such as sodium or potassium, iii) ammonium R,R',R"N + - with R, R' and R", which may be identical or different, representing a hydrogen atom, a (C 1 -C 6)alkyl group optionally substituted in particular with a hydroxyl group, such as hydroxyethyl; iv) and a cationic counterion derived from aqueous ammonia, monoethanolamine, diethanolamine, triethanolamine and 3-amino-1,2-propanediol, and basic amino acids such as lysine, arginine, sarcosine, ornithine and citrulline; • m + n is greater than or equal to 1; • m is such that the degree of substitution of the polysaccharide with one or more aldehyde groups (DS), which is greater than or equal to 0, in particular m is within the range of from 0 to 2, more particularly ranging from 0.001 to 1.8, preferably ranging from 0.005 to 1.5; • n is such that the degree of substitution of the polysaccharide with one or more carboxylic groups (DS(COOX)) is greater than 0, in particular less than or equal to 2, preferably between 0.001 and 1.5, inclusive; • p is such that the degree of substitution of the polysaccharide with one or more amine groups (DS(R 1 R 2 N)) is greater than 0, in particular less than or equal to 2, preferably between 0.001 and 1.5, inclusive. • P represents a polysaccharide chain constituted of monosaccharides comprising 5 carbon atoms or more than 5 carbon atoms, preferably 6 or more than 6 carbon atoms and more particularly 6 carbon atoms; • R 1 and R 2 represent a hydrogen atom; • X represents a hydrogen atom, a cationic counterion, in particular chosen from i) alkali metal, ii) alkaline earth metal, such as sodium or potassium, iii) ammonium R,R',R"N + - with R, R' and R", which may be identical or different, representing a hydrogen atom, a (C 1 -C 6)alkyl group optionally substituted in particular with a hydroxyl group, such as hydroxyethyl; iv) and a cationic counterion derived from aqueous ammonia, monoethanolamine, diethanolamine, triethanolamine and 3-amino-1,2-propanediol, and basic amino acids such as lysine, arginine, sarcosine, ornithine and citrulline; • m + n is greater than or equal to 1; • m is such that the degree of substitution of the polysaccharide with one or more aldehyde groups (DS), which is greater than or equal to 0, in particular m is within the range of from 0 to 2, more particularly ranging from 0.001 to 1.8, preferably ranging from 0.005 to 1.5; • n is such that the degree of substitution of the polysaccharide with one or more carboxylic groups (DS(COOX)) is greater than 0, in particular less than or equal to 2, preferably between 0.001 and 1.5, inclusive; • p is such that the degree of substitution of the polysaccharide with one or more amine groups (DS(R 1 R 2 N)) is greater than 0, in particular less than or equal to 2, preferably between 0.001 and 1.5, inclusive.