RED chemiluminescence corresponding to about 40 kcal/mol., and attributable to the forbidden transition 1Σ+g → 3Σ−g of the oxygen molecule, has been observed in three recent investigations1–3, but no explanation of the effect has been presented. These involve organic hydroperoxides or oxidations with hydrogenperoxide and also the reaction of hydrogen peroxide with HClO. In contrast with this, decompositions of hydrogen peroxide cataclysed by copper ions or by platinum show no trace of the effect, and in the photochemical decomposition of ozone ground state oxygen molecules are formed with vibrational energies up to 70 kcal/mol. (ref. 4). The state of newly formed oxygen molecules is evidently not determined solely by energy factors. Since in the thermal decomposition of an organic peroxide to a ketone (or quinine) about 70 kcal/mol. is liberated it is very possible that the ketosis (K) is formed in its triplet state. The spin-allowed reaction 3K + 3O2 → 1K + 1O2 would then account for the formation of singlet excited oxygen molecule.