A method for strengthening and repairing keratin fibres including providing a first—；and a second crosslinking composition, wherein the first—；crosslinking composition comprises an at least bi-functional Brønsted-base and the second—；crosslinking composition comprises—；an at least bi-functional organic acid, and a cosmetically acceptable carrier； applying the first—；as well as the second—；crosslinking composition to keratin fibres； and optionally mixing the first—；crosslinking composition into a commercially available hair coloring or hair bleaching composition.
