To evaluate the effects of fluoride on the performance of molten carbonate fuel cells (MCFCs), bench-scale cell tests and half-cell experiments have been performed with fuel gas containing HF. The cell voltage does not change due to the addition of HF. According to the following reaction, most of the added HF reacts with carbonate ion to form fluoride ion, which accumulates in the electrolyte. 2 HF+ CO 3 2−→2 F −+ H 2 O+ CO 2 Hence, the carbonate loss and the decrease of the electrolyte volume have a negative effect on the cell-life expectancy. Moreover, a large number of fluoride ions causes corrosion on the cathode current collector. However, the MCFCs can generate electricity even if the electrolyte contains 40 mol% of LiF. There are no obvious effects of fluoride ion on the anode and cathode reaction.