The effect of non-Faradaic electrochemical modification of catalytic activity, or in situ controlled promotion, was investigated during ethylene epoxidation on Ag films deposited on beta "-Al2O3, a Na+ conductor, at temperatures 240 to 280 degrees C and 500 kPa total pressure in the presence of chlorinated hydrocarbon moderators. The beta "-Al2O3 support permits controlled and reversible potentiostatic introduction of various levels of sodium on the Ag catalyst surface. It was found that sodium coverages up to 0.03 enhance the rate of epoxidation without affecting significantly the rate of complete oxidation. The promotion index of sodium for ethylene epoxidation is up to 40. Maximum selectivity to ethylene oxide (88%) is obtained for theta(Na) = 0.03 and 1 ppm dichloroethane. The promoting and synergistic action of sodium and chlorine is discussed on the basis of previous in situ controlled promotion studies and the prevailing ideas about the mechanism of ethylene epoxidation.