The effect of electrochemical promotion (EP) or non-faradaic electrochemical modi. cation of catalytic activity (NEMCA) was studied in the catalytic reaction of the total oxidation of propane on Pt and Rh. lms deposited on Y2O3-stabilized- ZrO2 (or YSZ), an O2- conductor, in the temperature range 420 - 520 degrees C. In the case of Pt/YSZ and for oxygen to propane ratios lower than the stoichiometric ratio it was found that the rate of propane oxidation could be reversibly enhanced by application of both positive and negative overpotentials ("inverted volcano'' behavior), by up to a factor of 1350 and 1130, respectively. The induced rate increase Delta r exceeded the corresponding electrochemically controlled rate I/2F of O2- transfer through the solid electrolyte, resulting in absolute values of the apparent faradaic efficiency Lambda= Delta r/( I/2F) up to 2330. The Rh/YSZ system exhibited similar EP behavior. Abrupt changes in the oxidation state of the rhodium catalyst, accompanied by changes in the catalytic rate, were observed by changing the O-2 to propane ratio and catalyst potential. The highest rate increases, by up to a factor of 6, were observed for positive overpotentials with corresponding absolute values of faradaic efficiency Lambda up to 830. Rate increases by up to a factor of 1.7 were observed for negative overpotentials. The observed EP behavior is explained by taking into account the mechanism of the reaction and the effect of catalyst potential on the binding strength of chemisorbed reactants and intermediates and on the oxidative state of the catalyst surface.