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In this study, we have investigated, for the very first time, the oxygen electrode reactions on Ag/YSZ electrochemical catalysts both in oxygen and under reaction conditions compatible with the ethylene epoxidation reaction. Electrochemical Impedance Spectroscopy (EIS) in combination with in-situ Raman spectroscopy and catalytic activity measurements were used to identify and understand the main oxygen reaction pathways. The results obtained suggested that the rate limiting step under an O2 reaction atmosphere (at 300 °C) is the O2 adsorption/dissociation process on the Ag catalyst-electrode. In addition, the polarization resistance increased with time under the presence of O2. This was attributed to the formation of Ag2O on the catalyst surface or near surface, which limits the oxygen electrode reactions. Finally, we observed that the addition of ethylene in the feed stream hinders the electrode reaction, due to its competitive chemisorption with oxygen on Ag. These results give new insights into the design of selective Ag/YSZ catalyst for ethylene epoxidation. Keywords: Electrochemical Impedance Spectroscopy (EIS), Oxygen electrode reaction, Ag/YSZ, Ethylene epoxidation