The present disclosure relates to kits and methods, which are particularly useful for repairing, strengthening, and protecting hair from damage. The kits and methods employ one or more booster compositions, which can be added to a separate hair-treatment composition for application to the hair, or can be applied individually to the hair in a sequence with a separate hair-treatment composition. The booster compositions include: at least 0.5 wt. % of at least one amino acid or amino sulfonic acid, and/or a salt thereof； at least 0.5 wt. % of at least one non-polymeric mono, di, or tricarboxylic acid, and/or a salt thereof； and water. Hair treated with the kits and methods exhibit desirable cosmetic properties such as smoothness, gloss, improved combability, and enhanced strength and elasticity.
