The　photochemical　oxidation　of　N,N-diacyl-1,4-dihydropyrazines　was　investigated　by　irradiation　from　a　medium-pressure　mercury　lamp.　The　main　products　of　the　photooxygenation　were　isolated　and　unambiguously　confirmed　by　H-1　NMR　spectra,　C-13　NMR　spectra,　H-H,　C-H　correlation　spectra,　high-resolution　mass　spectrometry,　and　single-crystal　x-ray　diffraction　analysis.　The　complicated　NMR　spectra　of　main　products　were　studied　by　variable-temperature　NMR　experiments.　The　mechanism　of　the　photooxygenation　of　N,N-diacyl-1,4-dihydropyrazines　is　suggested　to　be　a　[2+2]　cycloaddition　of　oxygen　to　the　double　bond.　[GRAPHICS]