The electrochem. promotion of RuO2 catalyst films, deposed onto yttria stabilized zirconia solid electrolyte (YSZ), was studied for the gas phase combustion of ethylene. Two order of magnitude increase in the reaction rate was obtained during anodic polarization of the catalyst compared with the unpromoted rate under open circuit conditions. The rate increase was about three orders of magnitude higher than the rate of oxygen pumping to the catalyst. Work function measurements were used for the study of the interaction of the RuO2/YSZ interface and the RuO2/gas interface. A two step mechanism was obsd. for the electrochem. promotion. A 1st fast modification is due to the polarization of the RuO2/YSZ interface and a slower modification due to the backspillover of oxygen ions from the three phase boundary over the gas exposed catalyst surface. [on SciFinder (R)]