A cosmetic method for treating strands of hair comprising the subsequent steps of: placing the strands of hair to be treated between two substrates on which a first and second compositions capable of reacting together are applied, and implement for the same. The implement for carrying out the method may comprise a first and a second substrates in the form of strips of material that may be folded over each other such that at least one section of the strands of hair to be treated is sandwiched between the first and second compositions.