Investigation of fuel cell applications using fluoride electrolytes, especially alkaline and alkaline earth fluorides, is a very new subject. Actually, most of the fluorides can function as fuel cell electrolytes, but the alkaline and alkaline earth fluorides are specially interesting since in composite materials they exhibit excellent performance for fuel cell electrolytes. Among the alkaline earth fluoride fuel cells, the best performance was achieved thus far for the fuel cell using the NaF-BaF2-Al2O3 electrolyte and LiNiO2 anode supported technique: it reached a power density of about 110 mW cm(-2) at a current density of 250 mA cm(-2) at 750 degreesC. Both proton and oxygen ionic conduction may exist in the alkaline earth fluorides since, during the fuel cell operation, water was observed at both the anode and the cathode, but, in most cases, water was formed at the cathode side mainly. Thus, proton conduction predominates, resulting in excellent fuel cell performances. A possible ionic transport mechanism is also discussed. (C) 2000 Elsevier Science B.V. All rights reserved.