High-temperature oxide melt solution calorimetry using sodium molybdate (3Na2O·4MoO3) solvent at 973 K was performed for the Na3xRE0.67–xTiO3 (RE = La, Ce) perovskite series. The enthalpies of formation of lanthanum perovskites from oxides (La2O3, Na2O, TiO2), are –107.25 ± 2.56, –93.83 ± 6.06, –80.68 ± 5.93, and –33.49 ± 4.26 kJ/mol and enthalpies of formation from elements are –1614.05 ± 5.37, –1596.44 ± 7.68, –1594.03 ± 7.58, and –1577.56 ± 6.36 kJ/mol for Na0.459La0.522Ti0.999O3, Na0.454La0.523Ti0.994O3, Na0.380La0.567Ti0.980O3, and La0.692Ti0.979O3, respectively. The enthalpies of formation of cerium perovskites are –99.98 ± 5.78 and –45.78 ± 3.30 kJ/mol from oxides (Ce2O3, Na2O, TiO2), and –1611.34 ± 6.90 and –1602.06 ± 2.72 kJ/mol from elements for Na0.442Ce0.547Ti0.980O3 and Ce0.72Ti0.96O3. The A-site defect perovskites become more stable relative to oxide components as sodium contents increase. Na0.5Ce0.5TiO3 and Na0.5La0.5TiO3 could be considered as thermodynamically stable end-members in natural loparite minerals, in which these end-members are in solid solution with CaTiO3 and other components.