Methionine (Met) is an essential sulfur-containing amino acid, sensitive to oxidation. The oxidation of Met can occur by numerous pathways, including enzymatic modifications and oxidative stress, being able to cause relevant alterations in protein functionality. Under UV radiation, Met may be oxidized by direct absorption (below 250 nm) or by photosensitized reactions. Herein, kinetics of the reaction and identification of products during photosensitized oxidation were analyzed to elucidate the mechanism for the degradation of Met under UV-A irradiation using pterins, pterin (Ptr) and 6-methylpterin (Mep), as sensitizers. The process begins with an electron transfer from Met to the triplet-excited state of the photosensitizer (Ptr or Mep), to yield the corresponding pair of radicals, Met radical cation (Met•+ ) and the radical anion of the sensitizer (Sens•- ). In air-equilibrated solutions, Met•+ incorporates one or two atoms of oxygen to yield methionine sulfoxide (MetO) and methionine sulfone (MetO2 ), whereas Sens•- reacts with O2 to recover the photosensitizer and generate superoxide anion (O2 •- ). In anaerobic conditions, further free-radical reactions lead to the formation of the corresponding dihydropterin derivatives (H2 Ptr or H2 Mep).