A method of decreasing cysteic acid formation in keratin fibers, especially human hair, during a chemical treatment such as permanent shaping and colouring or bleaching and especially oxidative colouring. A composition comprising at least one diamide compound is applied. Preferably, the diamide compound is selected from compounds according to the general structurewherein R1 is a linear or branched, saturated or unsaturated alkyl chain with 1 to 12 C atoms which may be substituted with hydroxy and/or alkoxy groups, preferably R1 is linear or branched, saturated or unsaturated alkyl chain with 1 to 12 C atoms which may be substituted by 1 to 3 substituents selected from a hydroxy group and C1 to C6 alkoxy group, more preferably R1 is a unsubstituted alkyl group with 1 to 12 C atoms, and alkyl group with 2 to 12 C atoms substituted by one or two hydroxyl groups, by one alkoxy group with 1 to 6 C atoms or by one hydroxyl and one alkoxy group with 2 to 6 C atoms, R2 is linear or branched alkyl chain with 1 to 5 C atoms, preferably linear or branched alkyl chain with 2 to 5 C atoms and more preferably an alkyl chain with 2 to 3 C atoms, and R3 linear or branched, saturated or unsaturated alkyl chain with 1 to 22 C atoms, preferably linear or branched, saturated or unsaturated alkyl chain with 11 to 22 C atoms.
