The thermal behaviours of La(NO3)(3) . 6H(2)O and La(OH)(2)NO3 . H2O have been reconsidered using temperature-dependent X-ray powder diffraction and thermogravimetric analysis. For the neutral lanthanum nitrate, the decomposition schemes are atmosphere dependent. For a water-vapour pressure greater than 4.7 Torr, five crystalline phases and one amorphous form have been displayed during the thermal decomposition to the anhydrous phase, while for P-H2O lower than 4.3 Torr, only three crystalline phases and one amorphous form have been identified. Depending on the P-H2O range, two polymorphic varieties of the tetrahydrated phase have also been obtained. Above 350 degrees C, the neutral and basic lanthanum nitrates have the same behaviour, and yield, in inert atmosphere, LaONO3, La3O4NO3 and La2O3 successively. In air LaONO, is strongly reactive with regard to CO2 to give the monoclinic form of La2O2CO3. The crystal structure of the tetragonal oxide nitrate (a=4.127(4) Angstrom, c=9.239(1), S.G. P4/mmm) is described, as well as the microstructural properties of La2O3 dominated by structural imperfections.