The stability of platinum and nickel (bare and fluorinated) electrodes was investigated for use as contamination-free reference electrodes in electrolytic production of NF3 using Ni as an anode in NH4F center dot 2HF melt. Though the platinum reference electrode showed stable potentials, thermodynamic analysis revealed that it is inappropriate as a reference electrode in long-term electrolysis using nickel anode because the potential depends on the concentration ratio of Ni3+/Ni2+. The stability of the bare nickel electrode was poor; however, an anodically and a thermally fluorinated nickel electrode showed excellent stability at 0.073 +/- 0.001 V at 373 K with good reproducibility. It is therefore concluded that the fluorinated nickel electrodes are useful as reference electrodes for electrolytic production of NF3 using Ni as an anode in the NH4F center dot 2HF melt. The equilibrium potential (0.073 +/- 0.001 V) obtained experimentally did not agree with the theoretically calculated potentials of NiF2/Ni (-0.178 V) nor NiO/Ni (0.164 V) at 373 K, and it was speculated that the potential might be determined by nickel oxyfluoride, NiOxFy, formed on the surface of nickel electrode. (c) 2007 The Electrochemical Society.