It was found that the catalytic activity of Pt for CO oxidation can be markedly and reversibly affected by depositing polycrystalline Pt films on beta"-Al2O3, a Na+ conductor, and applying external potentials to supply or remove Na to or from the Pt catalyst surface. The change in the rate of CO oxidation is typically 10(3)-10(5) times larger than the rate of supply or removal of Na. The use of the beta"-Al2O3 solid electrolyte supports permits precise in situ control of the Na coverage on the Pt surface. Sodium coverages of 0.02 cause up to 600% steady-state increase in the rate of CO oxidation under CO-rich conditions. The promoting effect is due to enhanced oxygen chemisorption on the Pt surface. Higher (>0.06) Na coverages poison the rate severely and reversibly due to the formation of a CO-Na-Pt surface complex. Rate oscillations can be reversibly induced or stopped and their frequency can be controlled by controlling the catalyst potential V(WR) and average work function ePHI.