The relation between the catalyst potential and the catalytic performance has been investigated in the gas-phase combustion of propylene with oxygen over rhodium catalysts at 375 DegC. The rhodium catalyst, deposited on yttria-stabilized zirconia (YSZ) solid electrolyte, also served as working electrode in the electrochem. cell. Under open-circuit conditions, the measured catalyst potential was found to be a sensitive indicator of the oxidn. state of the rhodium catalyst, which influences the catalytic reaction rate dramatically and depends strongly both on the method of catalyst film prepn. and on the compn. of the reacting gas mixt. In turn, under closed-circuit conditions, the applied catalyst potential is a convenient tool to maintain the catalyst in its more active, reduced form and to control its catalytic performance. The activity of at. oxygen at the three-phase boundary (tpb) during open-circuit catalytic reaction was estd. from solid electrolyte potentiometric (SEP) measurements, in good agreement with the av. surface oxidn. state obtained from XRD and XPS analyses. O/Rh at. ratios higher than stoichiometric were found by XPS at the outer surface of the catalysts suggesting a strong open circuit O2- spillover due to strong metal support interactions (SMSI) and a concomitant extension of the elec. double layer to the gas-exposed catalyst surface, similarly to emersed electrodes in aq. electrochem. Applying potentials up to several hundreds of mV, highly nonfaradaic promotion of propylene combustion was achieved. Electrochem. promotion of catalysis (EPOC) was most efficient at stoichiometric gas compn., i.e., close to the limit of surface redn., and with the catalyst exhibiting the smallest O2- spillover population at open-circuit conditions.