Sodium vapor was released in darkness on the downleg of a rocket trajectory, from 150 to 125 km. An intense headglow was observed surrounding the rocket, followed by a short-lived afterglow disappearing within 0.2-0.5 s. The glow is interpreted in terms of a chemiluminous reaction involving sodium dimers Na2. The dimers react with atomic oxygen either in a single step, Na2 + O→Na *+ NaO, or in two steps, Na2+O→Na+NaO and NaO+O→Na*+O2, with an effective bimolecular rate constant of 6¿10-11 cm3 s-1. The Na2-O reaction could be involved in the natural sodium nightglow, but it is difficult to explain the required fast resupply of dimers from monomers.