It is a method for determining a user-specific hair treatment under determination and the inclusion of the damage to degree of hair is made available. For this purpose, first of all with the aid of near infra-red - and / or infrared spectra of the keratin fibers of an individual the content of oxidation product of an amino acid is determined, and by means of a degree of damage to a calibration model derived. On the basis of the determined degree of damage to the individual treatment instruction is output.
