The complexation of lanthanides (Nd³⁺ and Eu³⁺) with glutaroimide-dioxime (H₂L), a cyclic imide dioxime ligand that has been found to form stable complexes with actinides (UO₂²⁺ and NpO²⁺) and transition metal ions (Fe³⁺, Cu²⁺, etc.), was studied by potentiometry, absorption spectrophotometry, luminescence spectroscopy, and microcalorimetry. Lanthanides form three successive complexes, M(HL)²⁺, M(HL)L, and M(HL)₂ ⁺(where M stands for Nd³⁺/Eu³⁺ and HL⁻ stands for the singly deprotonated ligand). The enthalpies of complexation, determined by microcalorimetry, show that the formation of these complexes is exothermic. The stability constants of Ln³⁺/H₂L complexes are several orders of magnitude lower than that of the corresponding Fe³⁺/H₂L complexes but are comparable with that of UO₂²⁺/H₂L complexes. A structure of Eu³⁺/H₂L complex, identified by single-crystal X-ray diffractometry, shows that the ligand coordinates to Eu³⁺ in a tridentate mode, via the two oxygen atoms of the oxime group and the nitrogen atom of the imide group. The relocation of protons of the oxime groups (−CH=N−OH) from the oxygen to the nitrogen atom, and the deprotonation of the imide group (−CH−NH−CH−) result in a conjugated system with delocalized electron density on the ligand (−O−N−C−N−C−N−O−) that forms strong complexes with the lanthanide ions.