LiNaSO4-Al2O3 ceramic composites were prepared for use as electrolytes in H-2/O-2 fuel cells. TG/DTA, XRD, and FT-IR studies on the structure and phase composition of the materials indicated no obvious chemical reaction between LiNaSO4 and alpha-Al2O3 during heat treatment up to 800 degrees C. The concentration of surface defects increased because of addition of alpha-Al2O3, which enhanced the ionic conduction of the materials. When a proper amount of alpha-Al2O3 was added, the conductivity of LiNaSO4-Al2O3 reached 10(-2) S/cm below the phase transition temperature (515 degrees C) of LiNaSO4 and 10(-1) S/cm above it. H-2/O-2 fuel cells using LiNaSO4-Al2O3 as an electrolyte showed an open circuit voltage of 1.3 V and a current density of 280 mA/cm(2) at 0.35 V terminal voltage. However, the rapid degradation of the cell performance implies that the materials may be unstable in the operational condition of fuel cells.