Abstract: The processes of the autooxidation and photooxidation of tetrahydrobiopterin (Н₄Bip) (a coenzyme present in a three- to fivefold excess in vitiligo) were studied in the context of vitiligo pathology and treatment. The study of the kinetics of Н₄Bip autooxidation and analysis of the reaction products via high-performance liquid chromatography (HPLC) demonstrated that autooxidation was intensive at a rate constant of 1 × 10^–3 s^–1 with the formation of dihydrobiopterin, dihydropterin, and their oxidized derivatives. Analysis of the autooxidation data led to a new conclusion that the oxidation of the excess of Н₄Bip in melanocytes obviously triggers an autocatalytic cycle of the synthesis of the excess of hydrogen peroxide (Н₂О₂). This, in turn, activates interferon-inducible GTP cyclohydrolase, which synthesizes an excess of Н₄Bip. The autocatalytic cycle of excess Н₂О₂ synthesis apparently underlies the pathology of vitiligo. The excess Н₂О₂ is also partly spent to activate the immune system. The autocatalytic cycle can be broken via the conversion of Н₄Bip into dihydropterin dimers during its UV photooxidation. The kinetics of Н₄Bip was studied, the reaction products were identified, and the quantum yields of the formation of dihydropterin dimers were calculated. The action spectrum of UV radiation was constructed based on the quantum yield data. It shows that the range of 300–325 nm is efficient for vitiligo phototherapy.