Abstract6-Formylpterin is one of the oxidized pterins that accumulate in the white skin patches of patients affected by vitiligo. Studies of photoinduced generation of hydrogen peroxide (H 2 0 2 ) and superoxide anion radical (0 2 · ) by formylpterin in acidic and alkaline aqueous solutions at 350 nm and room temperature have been performed. The photochemical processes were followed by UV-V1S spectrophotometry and enzymatic methods for H 2 0 2 and 0 2 *~determination. When formylpterin is irradiated a red intermediate, that has been proposed to be 6-carboxy-5,8-dihydropterin, is generated in an 0 2 -independent process. In the presence of air that intermediate reacts with 0 2 to yield H 2 0 2 and 6-carboxyptcrin. In anaerobic conditions it undergoes a thermal 0 2 -independent reaction which leads to a non-identified product without generation of H 2 0 2 . The rate constants of such a process were determined to be 0.011 min 1 and 0.044 min 1 at pH 5.4 and 10.5, respectively. (0 2 · ), is also generated during photooxidation of formylpterin, probably as a consequence of the electron transfer from the red intermediate to 0 2 . The biomedical importance of photoinduced generation of H 2 0 2 and 0 2 · is discussed.