This paper describes a laboratory study into the chemical pathways by which Na+ is converted to Na in the upper atmosphere. The termolecular clustering reactions of Na+ with N2, O2, and CO2 were studied in a low-temperature fast flow reactor coupled to a quadrupole mass spectrometer. This yielded k(Na++N2+He,93--255 K)=(1.20¿0.13)¿10-30(T/200 K)-(2.20¿0.09), k(Na++O2+He,93--130 K)=(5.20¿2.62)¿10-31(T/200 K)-(2.64¿0.74), k(Na++CO2+He,158--300 K)=(9.05¿1.38)¿10-30(T/200 K)-(2.84¿0.48), where the units are cm6 molecule-2 s-1 and the stated errors are a combination of the 2&sgr; standard errors in the kinetic data and the systematic errors in the temperature, pressure, and flow rates. It was then shown that atomic O will ligand switch with Na.N2+ but not with Na.CO2+, and that the former reaction proceeds essentially at the Langevin collision frequency. The neutralization of Na+ in the upper atmosphere is therefore rather complex. The first step is formation of the Na.N2+ ion from the recombination of Na+ with N2. This cluster ion can then either switch with CO2, which leads to a stable cluster ion that will undergo dissociative electron recombination to form Na; or switch with atomic O, which reforms Na+. The result of this is that the lifetime of Na+ changes very rapidly from more than a day above 100 km to just a few minutes at 90 km. Furthermore, the rate of neutralization is largely independent of the electron concentration. A simple model describing the conversion of Na+ to atomic Na in a descending sporadic E layer demonstrates that this ion-molecule mechanism appears to fulfil many of the major criteria for producing sporadic sodium layers. ¿ 1998 American Geophysical Union