A spike peak superimposed on the oxygen reduction wave, which usually is not evident under atmospheric conditions, was observed on gold electrodes in both (Li + Na)CO3 and (Li + K)CO3 eutectic melts under pressurized conditions. It has been analyzed by cyclic voltammetry as a function of carbonate composition, total oxidant gas pressure, oxidant gas composition and cathode electrode material. The spike peak appears distinctly in (Li + Na)CO3 melts, because its peak potential in (Li + Na)CO3 is located 30 mV more negative than in (Li + K)CO3. The charge represented by the spike peak was found to be about 5.1 mu C/cm(2), corresponding to less than a monolayer quantity (theta similar to 1/100). The spike current was proportional to the scan rate, indicating a surface redox reaction associated with the oxygen reduction process. Considering the electron-transfer mechanism of oxygen reduction, we discuss this phenomenon as an electrochemical adsorption/desorption step of an intermediate oxygen species adsorbed on a bare gold surface.