The effects of metallic impurities added and dissolved from the structure of the cell on the current efficiencies for NF3 formation and the overall generation of anode gas, and on the consumption of the nickel anode were studied in molten NH4F . 2HF at 120 degrees C by galvanostatic electrolysis. A mixed gas composed of NF3 and N-2 with a small amount of N2F2, N2F4, N2O, and O-2 was liberated at the nickel anode during electrolysis at 25 mA/cm(2). The current efficiencies for NF3 formation and the overall generation of anode gas under steady-state conditions depended on the structural material, i.e., the current efficiency of the copper cell was high compared with those of the steel and nickel cells. On the other hand, the current loss caused by the anodic dissolution df nickel in the copper cell was larger than that in the steel and nickel cells. Water in the melt retarded not only the anode consumption but also the current efficiency for NF3 formation. Although the addition of complexes such as NH4NiF3 and (NH4)(3)FeF6 in the melt was effective for minimizing the anode consumption, the NF3 formation was affected seriously Therefore, we estimate that the allowable contents of Ni2+ and Fe3+ in the melt should be no more than 0.06 and 0.03 mole percent, respectively.