Chemiluminescence and resonance fluorescence of FeO has been observed upon release of iron carbonyl into the atomic oxygen-rich region of the upper atmosphere. The FeO molecules are formed in a fast reaction between IC and atomic oxygen in the mixing zone surrounding the rocket. FeO does not persist to form a long-lasting wake, apparently since it reacts with another O to give Fe and O•. The upper atmosphere can be a convenient environment to study the formation, destruction, and radiation characteristics of some unstable molecular species that in the laboratory would suffer from the presence of walls, higher collision rates, or insufficient optical path. The molecule Fe0 is a case in point. Its emission spectrum has been observed in a flame of an iron arc in air, or when iron carbonyl is introduced into a flame [see, e.g., Pearse and Gaydon, 1950; Mavrodineanu and Boiteux, 1965]. However, little is known on the reaction of Fe0 with atomic oxygen or the resonance fluorescence emission when the molecule is irradiated. The irradiation effects particularly can be studied in the upper atmosphere.