Use for the treatment of keratin substances, particularly human keratin fibres such as hair of one or more compounds having formula (II) and addition salts and solvates thereof: Formula (II) in which: • R 1 and R 5 represent a hydrogen atom; • R 2 and R 4 , which may be identical or different, represent: - a hydrogen atom, - a linear or branched C 1 -C 10 alkyl substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy, and -NR 7 R 8 groups, - a linear or branched C 1 -C 10 alkoxy substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy and -NR 7 R 8 groups, - a -CO 2 H substituent, - a -COR 10 substituent, • R 3 represents: - a hydrogen atom; - a linear or branched C 2 -C 10 alkyl substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy and -NR 7 R 8 groups, - a linear or branched C 1 -C 10 alkoxy substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy, C 1 -C 4 hydroxyalkyl and -NR 7 R 8 groups; - a -CO 2 R 9 substituent, - a -COR 10 substituent, • ALK represents a methylene substituent, • R 6 represents a methyl substituent, • R 7 and R 8 , which may be identical or different, denote a hydrogen atom or a linear or branched C 1 -C 10 alkyl substituent optionally substituted with one or more hydroxyl, C 1 -C 10 alkoxy and amino groups; • R 9 and R 10 , which may be identical or different, denote a linear or branched C 1 -C 10 alkyl substituent optionally substituted with one or more hydroxyl, C 1 -C 10 alkoxy and amino groups; • Q - represents an anion or a mixture of organic or inorganic anions that ensure electroneutrality in compounds having formula (II). • R 1 and R 5 represent a hydrogen atom; • R 2 and R 4 , which may be identical or different, represent: - a hydrogen atom, - a linear or branched C 1 -C 10 alkyl substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy, and -NR 7 R 8 groups, - a linear or branched C 1 -C 10 alkoxy substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy and -NR 7 R 8 groups, - a -CO 2 H substituent, - a -COR 10 substituent, - a hydrogen atom, - a linear or branched C 1 -C 10 alkyl substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy, and -NR 7 R 8 groups, - a linear or branched C 1 -C 10 alkoxy substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy and -NR 7 R 8 groups, - a -CO 2 H substituent, - a -COR 10 substituent, • R 3 represents: - a hydrogen atom; - a linear or branched C 2 -C 10 alkyl substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy and -NR 7 R 8 groups, - a linear or branched C 1 -C 10 alkoxy substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy, C 1 -C 4 hydroxyalkyl and -NR 7 R 8 groups; - a -CO 2 R 9 substituent, - a -COR 10 substituent, - a hydrogen atom; - a linear or branched C 2 -C 10 alkyl substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy and -NR 7 R 8 groups, - a linear or branched C 1 -C 10 alkoxy substituent, optionally substituted by one or more hydroxyl, C 1 -C 10 alkoxy, C 1 -C 4 hydroxyalkyl and -NR 7 R 8 groups; - a -CO 2 R 9 substituent, - a -COR 10 substituent, • ALK represents a methylene substituent, • R 6 represents a methyl substituent, • R 7 and R 8 , which may be identical or different, denote a hydrogen atom or a linear or branched C 1 -C 10 alkyl substituent optionally substituted with one or more hydroxyl, C 1 -C 10 alkoxy and amino groups; • R 9 and R 10 , which may be identical or different, denote a linear or branched C 1 -C 10 alkyl substituent optionally substituted with one or more hydroxyl, C 1 -C 10 alkoxy and amino groups; • Q - represents an anion or a mixture of organic or inorganic anions that ensure electroneutrality in compounds having formula (II). <EOS>