Electro-pumping of Na or K to the surfaces of metal catalyst films can lead to very large and reversible changes in both catalytic activity and selectivity for a range of different reactions. This is illustrated with respect to the catalytic reduction of NO by CO and by propene over Pt, Rh and Cu, the combustion of propene over Pt, and the selective hydrogenation of acetylene over Pt. The results are interpreted in terms of alkali-induced changes to the adsorption state of reactants (NO reduction, propene combustion) or intermediate species (acetylene hydrogenation). At low alkali coverages, changes in catalyst potential scale linearly with the measured work function change. XP and Auger spectroscopy shows unambiguously that the changes in catalytic behaviour are due to changes in the surface concentration of the alkali. These electron spectroscopic data, supplemented by X-ray absorption spectra, show that the chemical state of the alkali promoter is dependent on the reactive gas environment. The fundamental insight provided by electrochemical promotion experiments can be used to guide the synthesis of practical dispersed catalysts.