Important contributions to the λ5577 dayglow may be made by dissociative recombination of O2+ and by photodissociation of O2 Both contributions are evaluated for a range of models corresponding to the uncertainties in the relevant cross sections. Available airglow data give an upper limit of 10 kilorayleighs on the noon zenith intensity due to both mechanisms combined. Other possible sources of excitation are also considered. Recombination of atomic oxygen, the mechanism responsible for the nightglow, does not contribute significantly above 120 km. Fluorescence and collisions with thermal electrons are also negligible, but collisions with energetic photoelectrons may be important. Abstract Important contributions to the λ5577 dayglow may be made by dissociative recombination of O2+ and by photodissociation of O2 Both contributions are evaluated for a range of models corresponding to the uncertainties in the relevant cross sections. Available airglow data give an upper limit of 10 kilorayleighs on the noon zenith intensity due to both mechanisms combined. Other possible sources of excitation are also considered. Recombination of atomic oxygen, the mechanism responsible for the nightglow, does not contribute significantly above 120 km. Fluorescence and collisions with thermal electrons are also negligible, but collisions with energetic photoelectrons may be important.