New photochemical studies of the reactivity of biopterin (BPT) and neopterin (NPT) in acidic (pH = 5.5) and alkaline (pH = 10.5) aqueous solutions at 350 nm and room temperature were performed. The photochemical properties of BPT are of particular interest because the photolysis of this compound takes place in the white skin patches of patients affected by vitiligo. The photochemical reactions were followed by UV/VIS spectrophotometry, HPLC, electrochemical measurement of dissolved O(2) and enzymatic methods for hydrogen peroxide (H(2)O(2)) and superoxide anion (O(2)(-)) determinations. When BPT or NPT are exposed to UVA radiation, a red intermediate, very likely 6-formyl-5,8-dihydropterin, is generated in an O(2)-independent process. That product is rapidly oxidized on admission of O(2) to yield 6-formylpterin and H(2)O(2). When the photolysis takes place in aerobic conditions, no additional pathways exist. On the other hand, in the absence of O(2), the intermediate generated is not stable and leads to the formation of many products. O(2)(-) is also generated during photo-oxidation of BPT and NPT. The quantum yields of reactant consumption depends on the O(2) concentration: the higher the O(2) concentration, the lower the quantum yields. This behavior is discussed in connection with the excited state of the pterins.