Spontaneous electrochemical processes occurring on Co and CoO in molten Li + K and Na + K carbonate eutectics saturated with a 0.9 O-2 + 0.1 CO2 atmosphere were investigated at 1000 K. It was shown that oxidation of Co to CoO proceeds via formation of an unstable compound in Li2CO3 + K2CO3. There is no indication that such a compound is formed in Na2CO3 + K2CO3. CoO undergoes further oxidation in molten carbonates producing LiCoO2 in Li2CO3 + K2CO3 and NaCoO2 (presumably) in Na2CO3 + K2CO3. 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.