Complexation of NpO2+ ions with glutaroimide dioxime (H2L), a cyclic imide dioxime ligand that has been shown to form strong complexes with UO22+ in aqueous solutions, was studied by absorption spectroscopy and microcalorimetry in 1.0 M NaClO4 aqueous solutions. NpO2+ forms two successive complexes, NpO2(HL)(aq) and NpO2(HL)(2)(-) (where HL- stands for the partially deprotonated glutaroimide dioxime ligand), with stability constants of log beta(111) = 17.8 +/- 0.1 and log beta(122) = 33.0 +/- 0.2, respectively. The complexation is both enthalpy- and entropy-driven, with negative enthalpies (Delta H-111= -52.3 +/- 1.0 kJ/mol and Delta H-122 = -96.1 +/- 1.4 kJ/mol) and positive entropies (Delta S-111 = 164 +/- 3 J/mol/K and Delta S-122 = 310 +/- 4 J/mol/K). The thermodynamic parameters suggest that, similar to complexation of UO22+, the ligand coordinates with NpO2+ in a tridentate mode, via the two oxygen atoms of the oxime groups and the nitrogen atom of the imide group. Density functional theory calculations have helped to interpret the optical absorption properties of the NpO2(HL)(2)(-) complex, by showing that the cis and trans configurations of the complex have very similar energies so that both configurations could be present in the aqueous solutions. It is the noncentrosymmetric cis configuration that makes the 5f -> 5f transition allowable so that the NpO2(HL)(2)(-) complex absorbs in the near-IR region.