The present invention relates to a process for treating keratin fibres, comprising the application to the fibres i) of at least one nucleophilic alkoxysilane polymer, in particular at least one aminoalkoxysilane polymer, and ii) at least one aliphatic-chain activated (thio)ester. The invention also relates to a cosmetic composition comprising ingredients i) and ii) and to a kit comprising ingredients i) and ii) for performing such a process. The process gives the treated damaged fibre a long-lasting hydrophobic surface state, the hydrophobicity effect being persistent after one or more shampoo washes, while at the same time affording a good cosmetic feel.
