The effect of electrochem. promotion of catalysis or nonfaradaic electrochem. modification of catalytic activity or electropromotion was studied for the model reaction of ethylene oxidn. 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. The catalytic activity of Rh for C2H4 oxidn. can be reversibly enhanced via anodic current or potential application by up to a factor of 80 and the increase in the oxidn. 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 electrochem. promoted catalyst state. The obsd. pronounced electrochem. 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 use of the electrochem. promotion of catalysis. [on SciFinder (R)]