The invention provides a method of strengthening the fibres of hair, the method comprising the sequential steps of: (i) washing the hair； (ii) soaking the washed hair in an aqueous treatment composition, and (iii) drying the soaked hair； characterised in that the aqueous treatment composition comprises at least 1% N-acetyl lysine (by weight based on the total weight of the composition). The invention also provides the use of an aqueous treatment composition comprising at least 1% N-acetyl lysine (by weight based on the total weight of the composition), for the strengthening of hair fibres.
