Hydrogen peroxide, as a sole reagent, is commonly used as an oxidizing agent in hair bleaching. By contrast, in the current study, a mixture of two oxidants, viz. ammonium persulfate and hydrogen peroxide was used to investigate the oxidation characteristics of cystine present in hair. The cystine in α-keratin protein underwent fast oxidation to produce two intermediate products, viz. cystine monoxide and cystine dioxide. Due to such rapid formation of the two intermediates, their characteristic ATR-FTIR peaks, such as 1071 cm^-1 for cystine monoxide and 1124 cm^-1 for cystine dioxide showed little or no change with increasing bleaching time. The ATR-FTIR peak (1041 cm^-1) of cysteic acid, the final product, increased linearly with square root of the bleaching time, and the oxidation rate of cystine by hydrogen peroxide and ammonium persulfate was very fast compared to the diffusion rate of hydrogen peroxide and ammonium persulfate into the hair. Therefore, the formation rate of cysteic acid depended on the diffusion rate of hydrogen peroxide and ammonium persulfate.