Thermodynamic properties of the natural calcium zeolites laumontite, leonhardite, dehydrated leonhardite (metaleonhardite), wairakite, and yugawaralite were studied by calorimetry in lead borate solvent at 975 K. Enthalpies of formation from the elements at 298 K are as follows: −7251.0 ± 8.5 kj/mol for laumontite, CaAl2Si4O12-4H2O; −7107.3 ± 5.6 kj/mol for leonhardite, CaAl2Si4O12 · 3.5H2O; −5964.3 ± 5.1 kj/mol for metaleonhardite, CaAl2Si4O12; −6646.7 ± 6.3 kj/mol for wairakite, CaAl2Si4O12-2H2O; and −9051.3 ± 10.4 kj/mol for yugawaralite, CaAl2Si6O16 · 4H2O. The value for leonhardite is in good agreement with early values from acid calorimetry (Barany 1961) but not with revised values from Hemingway and Robie (1977). The enthalpy of dehydration of leonhardite is 140.2 ±6.7 kj/mol, and the loss of one mole of H2O is associated with an endothermic effect of about 40 kI. Standard entropies, 5%, of wairakite [400.7 J/(molK)] and yugawaralite [609.8 J/(molK)] were derived from our new enthalpy data combined with reversed P-T phase equilibria (Liou 1971; Zeng and Liou 1982). The upper limit of wairakite stability, the univariant curve for equilibrium of wairakite with anorthite, quartz, and fluid, was calculated from these values of enthalpy and entropy. Good agreement between thermodynamic calculations and reversed phase equilibria supports the reliability of the new thermodynamic data.