The hydroxyl bands in the glow emanating from the earth's upper atmosphere are chemically excited. The two possible precursor reactions are O3+H→OH*+O2 and O2*+H→OH*+O. Arguments are advanced in this paper to show that the night-time variations of the emission intensifies stimulated by these two reactions might be characteristically different. The relative importance of the reactions can therefore be judged by a systematic observational study of the intensity variations.