The electrochemical reduction of O2 in molten carbonates occurs through two reactions: and The onset of these two processes has been investigated in molten , the ternary eutectic , and the two binary eutectics, and . Reaction [1] is found to start at slightly negative potentials relative to the reference electrode and depends on the supply of from the melt. The onset of reaction [2] occurs at more negative polarizations; both are affected by the solubility of O2 in the melt. The results for reaction [1] correlate well with the thermal dissociation of carbonates as a function of temperature and melt composition. Reaction [2] appears inhibited at lower temperatures but can be obtained reproducibly if the electrode is potentiodynamically cycled several times through the voltage sweep range; at elevated temperatures it occurs readily. The results are examined relative to the differing viewpoints advanced relative to the electrochemical reduction of oxygen in such molten electrolytes.