The effect of electrochemical promotion of catalysis or nonfaradaic electrochemical modification of catalytic activity or electropromotion was investigated for the model reaction of ethylene oxidation on sputter-coated Rh films. The thin (40 nm) Rh films were deposited on Y2O3-stabilized-ZrO2 (YSZ) and on YSZ coated with a thin porous TiO2 layer. It was found that the catalytic activity of Rh for C2H4 oxidation can be reversibly enhanced via anodic current or potential application by up to a factor of 80 and that the increase in the oxidation rate is up to 2000 times larger than, the rate of supply of O2- to the Rh catalyst-electrode. Smaller anodic currents cause periodic catalytic rate and potential oscillations. The TiO2 layer was found to enhance the open-circuit catalytic activity and to stabilize the electrochemically promoted catalyst state. The observed pronounced electrochemical promotion behavior is due to the anodically controlled migration (back spillover) of O2- species from YSZ to the Rh/gas interface and the concomitant destabilization, via repulsive lateral interactions, of the formation of surface Rh2O3. The electropromotion of such thin metal catalyst films with metal dispersion near 10% is of significant importance for the practical utilization of the electrochemical promotion of catalysis. (C) 2004 The Electrochemical Society.