The Ag-catalysed epoxidation of propene and ethene have been studied using an electrochemical promotion method involving supply or removal of potassium from the surface of the working catalyst. The two systems respond in very different ways to the addition of K, NO, Cl, and combinations of these modifiers, pointing to important differences in reaction mechanisms. The results suggest that in the case of propene epoxidation adsorbed alkali superoxides or peroxides may act as epoxidising agents, in addition to or instead of O-a. Control experiments with CaCO3, K-beta"Al2O3, and a Ag/K-beta"Al2O3 dispersed catalyst strongly suggest that support effects do not play a critically important part in determining the commonly observed low selectivity of propene epoxidation. Further conversion of propene epoxide is not an issue: the problem resides in the primary chemistry and probably hinges on the presence of allytic hydrogen atoms in the alkene. Postreaction X-ray photoelectron spectroscopy data suggest that the state of oxygenation of the Ag subsurface region is likely to be significant in determining the very different selectivities observed in ethene and propene epoxidation.