The thermal decompositions of K 2 La(NO 3 ) 5 • 2H 2 O and K 3 La 2 (NO 3 ) 9 have been studied under vacuum and under nitrogen by means of temperature-dependent X-ray diffraction and conventional thermal analysis. It is shown that the complexity of the decomposition processes arises from the formation of liquid phases. Interpretation of the complete decomposition scheme demanded the concurrent determination, by differential scanning calorimetry, of the phase relationships in the two binary systems LaONO 3 -KNO 3 and K 3 La 2 (NO 3 ) 9 -KNO 3 . In addition, the crystal structure of K 3 La 2 (NO 3 ) 9 has been refined from singlecrystal diffraction data (cubic symmetry, a)5(7066.31؍ A s , space group P4 1 32). In the diagram LaONO 3 -KNO 3 , the definite compound LaONO 3 • 1 3 KNO 3 has been identified. It undergoes a phase transformation at 310°C ( tr H 0 0079؍ J mol ؊1 ). The X-ray diffraction patterns of the two varieties have been indexed with orthorhombic symmetry, and, from cell dimension analogies, a crystal structure derived from the structure of LaONO 3 , with species like ''KNO 3 '' located between layers (LaO) n؉ n , is suggested. A polymorphic variety of LaONO 3 has been observed above 167°C. In the phase diagram K 3 La 2 (NO 3 ) 9 -KNO 3 a eutectic has been found at 0.906 and 250°C. The decomposition of K 2 La(NO 3 ) 5 • 2H 2 O takes place with the successive occurrence of the mixture of phases [K 3 La 2 (NO 3 ) 9 , KNO 3 ], the eutectic K 3 La 2 (NO 3 ) 9 • 9.62KNO 3 , the phase LaONO 3 • 1 3 KNO 3 and, surprisingly, the formation of cubic La 2 O 3 , which in turn transforms into the hexagonal variety at higher temperature. The decomposition of K 3 La 2 (NO 3 ) 9 proceeds through the successive formation of LaONO 3 • 1 3 KNO 3 and cubic La 2 O 3 . From a Rietveld refinement, it has been shown that the crystal structure of cubic La 2 O 3 [a ؍ 11.414(3) A s at 540°C] is isostructural with the cubic rate earth sesquioxides.