Human hair may be subjected to a range of potentially damaging processes, which can adversely affect the visual and mechanical properties of the fibre. In this study proteomic analysis has been evaluated as a potentially sensitive characterisation tool for assessing the effect of bleaching on the hair protein structure. As part of the method development the optimisation of fibre digestion, maximising protein yield and improving protein separation has been undertaken resulting in the development of a novel extraction technique. Location of the site of protein analysis along the fibre shaft has been demonstrated to influence the protein composition, with protein being lost with ageing. Similarly bleaching of hair resulted in a higher proportion of proteins being more easily solubilised, in particular the minor Type I keratins, K37, K38 and K40. A novel analytical procedure has been developed to probe the changes within individual proteins and identify and differentiate the cleavage sites. This has led to the characterisation of both naturally weak sites and sites of weakness attributed to the effects of bleaching. These sites have been quantified in terms of identifying the specific sequences along these weak zones that are more likely to be cleaved. The results show that no particular keratin is singled out for bleach-induced damage, although some are affected more than others (minor Type I keratins). Bleaching has been shown to damage all keratins at specific sites that differ for Type I and Type II keratins. An investigation into Bycoâ¢ C has shown that it can help reduce some of the damage caused by bleaching whilst maintaining the desired colour lightening properties sought with this kind of treatment.