The electrochemical promotion of RuO2 catalysts interfaced with yttria stabilised zirconia was studied for the combustion of ethylene. The complete oxidation of ethylene can be described by an Eley-Rideal type mechanism. In situ work function measurements were used to proof the mechanism. A linear dependence between the catalyst work function and the surface coverage of the only adsorbed reactant, chemisorbed oxygen, was found. Cyclic voltammetric measurements were used to study the RuO2/YSZ interface. The electrochemically active zone was located at the RuO2/YSZ interface and next to The three-phase boundary with only a small extension on the gas-exposed catalyst surface.