Two types of NiO electrode, either monocrystalline or obtained by oxidation of a Ni thin layer on Au in molten carbonates, were employed in these studies. The experiments were performed in molten Li + K and Na + K carbonate eutectics saturated with a 0.9 O-2 + 0.1 CO2 atmosphere at 1000 K. It was concluded that the oxidation of Ni proceeds predominantly by decomposition of an unstable compound in the Li2CO3 + K2CO3 melt and directly by evolution of CO2, in the Na2CO3 + K2CO3 melt. The measurements of the exchange currents and double-layer capacities showed that the reaction path of oxidation affects the effective area of NiO electrodes.