The density, molar volume, and surface tension of binary, ternary, and quaternary molten carbonate mixtures of Li2CO3-Na2CO3 and Li2CO3-K2CO3 containing alkaline earth (Ca, Sr, and Ba) carbonates were measured using a maximum bubble pressure technique. The addition of alkaline earth carbonates resulted in increases in the density and surface tension of the molten carbonate mixtures. The apparent molar volume of a component carbonate in mixtures can be derived from the density data, assuming additivity, and inversely be used to estimate the molar volume and density of mixtures within an error of 1%. The surface tensions of alkali-alkaline earth carbonates were proportional to the mole fraction of the added alkaline earth carbonate. (C) 2003 The Electrochemical Society.