The chemical composition and the electrochemical properties of the oxide layer formed on a nickel anode in molten CsF . NH4F . 4HF at 50 and 100-degrees-C were studied. Nickel compounds of high oxidation state such as NiF4, Ni3O4, and Ni2O3 were formed on the anode at potentials more positive than 3 V vs. the hydrogen evolution potential in the melt. NiO1+x (0 < x < 0.5), especially Ni3O4, was conductive and electrochemically active in contrast with the passive layer of divalent compounds (NiO and NiF2) formed at low potentials. It is estimated that these layers are less than several tens of angstroms thick depending on the polarization potentials and the time of electrolysis. It is also estimated that the thickness of these layers formed in molten CsF . NH4F . 4HF containing water is larger than that formed in a well-dehydrated melt.