The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA), or in situ controlled promotion was investigated during ethylene epoxidation on Ag films deposited on yttria-stabilized zirconia at temperatures from 240 to 300 degrees C and 500 kPa total pressure in the presence of chlorinated hydrocarbon moderators. It was found that the rates of epoxidation and complex oxidation change by a factor of 230 as the catalyst potential and work function are varied by 0.6 V and 0.6 eV, respectively. The change in the total reaction rate is typically a factor of 100 higher than the rate I/2F of electrochemical supply or removal of promoting oxide ions. The selectivity to ethylene oxide can be varied between 0 and 78% by varying the catalyst potential and dichloroethane partial pressure. At low catalyst potentials acetaldehyde becomes the main product with a selectivity up to 55%. The observed behavior is discussed on the basis of previous NEMCA studies and of the prevailing ideas about the mechanism of ethylene epoxidation.