The present invention relates to a process for treating keratin fibers, preferably hair, comprising the steps of: (i) applying to the keratin fibers a reducing composition comprising at least one disulfide and at least one thiol, wherein the weight ratio of disulfide/thiol is from 0.35 to 0.75, inclusive, preferably 0.4 to 0.7, and more preferably 0.45 to 0.65 (ii) optionally rinsing the keratin fibers (iii) oxidizing the keratin fibers and (iv) optionally rinsing and/or drying the keratin fibers. The present invention provides a new treatment process, in particular a permanent deformation process, for keratin fibers such as hair, which can provide superior cosmetic effects such as long-lasting deformation, sufficient curl efficiency, reduced hair degradation and non-malodor, in a relatively short period of time.
