The various electronically-excited states of oxygen atoms and molecules are briefly considered. Certain reactions involving some of these electronically-excited species are then discussed with reference to the experimental evidence and to the potential-energy surfaces on which they occur. In the case of the mercury-photosensitized formation of ozone from oxygen it is concluded that both experimental evidence and theoretical arguments point to the fact that the oxygen molecule initially formed is in an excitedstate. Consideration is also given to the mechanism of formation of O2* in the carbon monoxide flame and in other flames. The reactionsandare also discussed briefly.