An analysis of the possible dissociation of metastable oxygen molecules subject to constraints imposed by selection rules for molecular transitions, airglow observations, and atmospheric chemistry leads to the following conclusions. Dissociation of O2(b1&Sgr;g+) must produce a negligible number of oxygen atoms at all altitudes in the earth's atmosphere. However, if the dissociation cross section of O2(a1Δg) has a maximum value in the range 10-20 cm2 to 10-19 cm2, then the process O2(a1Δg)+hv→O2(C3Δu)→ O(3P)+O(3P) will constitute a significant, and potentially the major, source of odd oxygen in the uppermost stratosphere and mesosphere.