<p>Novel ternary complexes of light lanthanide(III) ions (La–Gd, except Pm) with niflumic acid and 1,10-phenanthroline as a coligand were successfully synthesized and characterized using elemental analysis, FTIR, UV–VIS as well as ¹H NMR spectroscopy, X-ray diffraction (XRPD), and thermogravimetric analysis. Based on these results, it was determined that the obtained complexes are hydrates and were synthesized with high yield. Although all complexes were synthesized under the same experimental conditions, they differ in stoichiometry.The use of DSC analysis allowed for the identification of both crystallization and coordination water in their composition. The spectroscopic data suggest that niflumate ions coordinate to Ln(III) ions through the carboxylate group in a bidentate mode.</p><p>The thermal decomposition pathways of the complexes were examined in air and nitrogen atmospheres through simultaneous TG/DTG–DTA analysis. It was found that the thermal decomposition of the complexes is a multistage process, varying depending on the metal ion and the atmosphere used. Identification of decomposition products released during thermal oxidation and pyrolysis of the complexes was performed using combined TG–FTIR–MS techniques. The EGA analysis during oxidative decomposition and pyrolysis showed that the gaseous products primarily included H₂O, CO₂, COF₂, and fragments from the partial decomposition of the organic ligands, as well as CO and pyridine in N₂.</p>