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 Bronsted-base and the acidic crosslinking composition omprises an at least bi-functional Bronsted-acid, and a cosmetically acceptable carrier comprising at least one fatty alcohol; 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 coloring or hair bleaching composition.