The effect of electrochemical promotion or nonfaradaic electrochemical modification of catalytic activity (NEMCA effect) was investigated for the first time on highly dispersed Pt catalyst (dispersion 0.2-1) supported on polycrystalline Au films which had been deposited on Y2O3-stabihzed ZrO2 (YSZ), an O2- conductor. The oxidation of ethylene to CO2 was chosen as a model reaction. It was found that the catalytic rate of ethylene oxidation could be reversibly enhanced by up to a factor of 5 via electrical current or potential application between the supported Pt/Au catalyst and a counter Au electrode also deposited on the YSZ ionic conductor. The increase in catalytic rate is typically a factor of 10(3) higher than the rate, I/2F, of electrochemical supply or removal of O2- to (from) the dispersed Pt/Au catalyst from (to) the solid electrolyte. Analysis of the time constant of the catalytic rate upon current application or interruption suggests that, as in previous electrochemical promotion studies with continuous metal film catalysts, the observed promotional phenomena are due to electrochemically controlled migration (spillover and backspillover) of anionic oxygen between the solid electrolyte and the metal catalyst surface and to the effect of this anionic oxygen on the chemisorptive bond strength of coadsorbed oxygen and ethylene.