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