Abstract: An improved molten carbonate fuel cell electrolyte wherein the inert support matrix is predominately LiAl0.sub.2 having crystalline structure with an average L/D greater than about 3 and surface area greater than about 5 sq. meters/gram and the carbonates composition is a ternary lithium-potassium-sodium or binary lithium-potassium or lithium-sodium carbonates composition having a thermal expansion of about 1.5 to about 3.5 .times. 10.sup.-5 inch/inch .degree. C. The improved molten carbonate fuel cell electrolyte of this invention provides fuel cell electrolytes capable of containing 50 to 70 wt. percent carbonate components to provide high power densities while providing a thin electrolyte which is resistant to physical damage on thermal cycling of the fuel cell. The electrolyte of this invention may also be reinforced with metallic alloys to provide additional strength characteristics.