Spontaneous electrochemical processes occuring on Co and CoO in molten Li + K and Na + K carbonate eutectics saturated with a 0.9 O 2 + 0.1 CO 2 atmosphere were investigated at 1000 K. It was shown that oxidation of Co to CoO proceeds via formation of an unstable compound in Li 2 CO 3 + K 2 CO 3 . There is no indication that such a compound is formed in Na 2 CO 3 + K 2 CO 3 . CoO undergoes further oxidation in molten carbonates producing LiCoO 2 in Li 2 CO 3 + K 2 CO 3 and NaCoO 2 (presumably) in Na 2 CO 3 + K 2 CO 3 . The oxidation processes increase the active area of the electrode as indicated by the measurements of exchange current densities, double-layer capacitances and impedances of electrodes.