A proton conductor, Ba3Ca1.18Nb1.82O9-alpha, has been used to electrochemically promote, via potential-controlled spillover of protons, a Pt catalyst-electrode for the model reaction of ethene oxidation at temperature 250-350 degreesC. The rate enhancement is up to 12 times that of the open-circuit rate and typically 10(3) times larger than the rate of proton supply to the catalyst. At the highest temperatures and most oxidizing conditions employed an up to threefold permanent electrochemical enhancement in catalytic rate was also observed. In situ a.c. impedance spectra reveal a buildup of sacrificial promoter, most likely OH formed by reaction of spillover protons with chemisorbed oxygen, under electrochemical promotion conditions. The observed pronounced electrophilic (partial derivativer/partial derivativeU less than or equal to 0) behavior, in conjunction with the reaction kinetics, is in good agreement with the recently established rules of electrochemical and classical promotion. (C) 2003 Elsevier Ltd. All rights reserved.