A method for strengthening and repairing keratin fibres including providing a caustic as well as an acidic crosslinking composition, wherein the caustic crosslinking composition comprises an at least bi-functional Brønsted-base and the acidic crosslinking composition comprises 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 fibres； and optionally mixing the caustic crosslinking composition into a commercially available hair colouring or hair bleaching composition.
