The catalytic activity of polycrystalline Pd films deposited on 8 mol% Y2O3-stabilized-ZrO2 (YSZ), an O2--conductor, can be altered reversibly by varying the potential of the Pd catalyst film via the effect of nonfaradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion. The complete oxidation of ethylene was investigated as a model reaction in the temperature range 290-360 degreesC and atmospheric total pressure. The rate of C2H4 oxidation can be reversibly enhanced by up to 45% by supplying O2- to the catalyst via positive current application. The steady-state rate change is typically 10(3)-10(4) times larger than the steady-state rate I/2F of electrochemical supply or removal of promoting oxide ions. The observed behaviour is discussed on the basis of previous NEMCA studies and the mechanism of the reaction.