The catalytic activity of polycrystalline Rh films deposited on 8 mol% Y2O3-stabilized ZrO2, (YSZ), an O2- conductor, can be altered dramatically and reversibly by varying the potential of the Rh catalyst film. The complete oxidation of ethylene was investigated as a model reaction in the temperature range 300 to 400 degrees C and at atmospheric total pressure. The rate of C2H4 oxidation can be reversibly enhanced by up to 100 times by supplying O2- to the catalyst via positive potential application (up to 1.5 V). This is the highest rate enhancement observed so far with in situ electrochemical promotion studies. The steady-state rate increase is typically 10(4) times larger than the steady-state rate of O2- supply to the catalyst. It was also found that varying the catalyst potential causes the appearance of the well-known compensation effect with an isokinetic point at 372 degrees C. As in previous studies of the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) the observed behaviour is due to the promotional action of back-spillover oxide ions which migrate from the YSZ solid electrolyte onto the catalyst surface under the influence of the applied potential. The back-spillover oxide ions are less reactive with C2H4 than normally chemisorbed oxygen and act as promoters by affecting the binding strength of chemisorbed oxygen and ethylene.