Thermodynamics of the Na1-xLixNbO3 system is investigated by high-temperature drop-solution calorimetry in molten 3Na(2)O-4MoO(3) solvent at 973 K. Standard molar enthalpies of formation are derived. The estimated heats of transition between hypothetical and stable structures, lithium niobate and perovskite for NaNbO3 and vice versa for LiNbO3 are -6 kJ/mol and -10 kJ/mol, respectively. X-ray diffraction studies at room temperature showed for 0 less than or equal to x less than or equal to 0.14 there are three phases based on different ordering of the perovskite type lattice: orthorhombic with a quadrupled reduced perovskite cell at 0 less than or equal to x less than or equal to 0.02, orthorhombic with a doubled reduced perovskite cell at 0.015 less than or equal to x less than or equal to 0.14, and rhombohedral at 0.08 less than or equal to x less than or equal to 0.13. There are two two-phase (morphotropic) regions with coexistence of the two orthorhombic phases at 0.015 less than or equal to x less than or equal to 0.02 and with the second orthorhombic phase coexisting with the rhombohedral phase at 0.08 less than or equal to x less than or equal to 0.13. A reproducible anomaly in specific heat at similar to600 K, not reported previously, has been observed in pure NaNbO3. Heat-capacity measurements confirm a phase transition at 553 K for 0.07 less than or equal to x less than or equal to 0.09. With increasing lithium concentration, a gradual disappearance of high-temperature phase transitions associated with tilting of oxygen octahedra has been observed.