XPS shows that the electropumping of Na to a metallic Rh film contacted with a solid electrolyte (Na beta " alumina) under potentiostatic conditions results in reversible spillover of the alkali from the electrolyte to the catalytically active surface. As the catalyst potential (V-WR) is decreased over a range of 2000 mV, the sodium coverage increases linearly from zero to similar to0.02 monolayers, while the work function (phi) decreases, also linearly, by 0.65 eV. Over this same regime of catalyst potential, under approximately stoichiometric conditions, both the activity and N-2 selectivity of the NO + CO reaction are strongly enhanced as Na is pumped to the Rh surface. The measured Na coverage, electrochemical behavior, and catalytic response of the system are fully reversible with catalyst potential. Our data are understandable in terms of the Na-induced dissociation of adsorbed NO, which is thought to be the rate-limiting step. The nature of electrochemical promotion, the reaction mechanism, the mode of promoter action, and the inequivalence of Delta phi and DeltaV(WR) are discussed.