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.