Methods and compositions for relaxing and/or straightening keratin fibres are disclosed. The method comprises i) simultaneously or sequentially applying to the keratin fibres a composition (A) and a composition (B); or ii) alternatively, applying to the keratin fibres a composition (D); and iii) heating the keratin fibres with a heating tool after both the composition (A) and the composition (B) are applied to the keratin fibres, or after the composition (D) is applied to the keratin fibres. The composition (A) is acidic and comprises at least one thiol-based reducing agent. The composition (B) comprises at least one compound chosen from compounds of formula (I): RO—C(O)-ALK-C(O)—OR′. The composition (D) is acidic and comprises at least one thiol-based reducing agent and at least one compound chosen from the compounds of formula (I).