High-level ab initio calculations were carried out on a series of K+center dot X cluster ions (X = O, O-2, N-2, CO2, H2O) and X center dot K+center dot Y ions. Rice-Ramsberger-Kassel-Markus theory was then used to estimate the rate coefficients for a series of recombination and ligand-switching reactions that govern the ion-molecule chemistry of K+ in the upper mesosphere and lower thermosphere. These rate coefficients were then included in an atmospheric model of potassium chemistry. The important result is that K+ forms weakly bound clusters with N-2, O-2, and O (the major atmospheric species), with binding energies between 10 and 22 kJ mol(-1). Even under atmospheric conditions (200 K and 10(-3) Tort), these cluster dissociate in less than 1 s. This prevents the formation by ligand-switching of the more stable CO2 and H2O clusters, which could then undergo dissociative recombination with electrons to produce K. The result is that K+ ions have a much longer lifetime against neutralization in the upper atmosphere than other metallic ions such as Na+ and Fe+.