Potassium, electrochemically supplied from K beta "-alumina to a silver thin film catalyst in the presence of ppm levels of NOx, strongly promotes the selectivity of ethene epoxidation. However, in the absence of gaseous NOx, alkali catastrophically suppresses both activity and selectivity. Addition of surface chlorine via ppm levels of ethylene dichloride further enhances the promotional effect of alkali + NO yielding the highest selectivity of all. The minimum necessary and sufficient conditions for the appearance of NOx promotion are submonolayer quantities of alkali on the metal surface, and ppm levels of gaseous NOx.