Analysis of the O I(&lgr;5577 A) airglow observations conducted on board the Atmosphere Explorer C satellite yields the following results when combined with simultaneous measurements of ion and neutral composition. A volume emission rate profile derived after sunset requires the production of an O(1S) atom in 8% of the O2 + recombinations. Photoelectron impact on atomic oxygen and the dissociative recombination of O2 + are important sources of O(1S) in the dayglow; however, the airglow measurements require a large additional production of O(1S) below 200 km in addition to these. The reaction N+O2 +→NO++O(1S) can provide the missing O(1S) source if the rate coefficient is near 2.5¿10-11 cm3 s-1.