Complexation of NpO2+ with dipicolinic acid (DPA) has been investigated in 1 M NaClO4 at 25 degrees C. Two complexes, NpO2(DPA)(-) and NpO2(DPA)(2)(3-), were identified, and the stability constants (log beta(1) and log beta(2)) were determined to be 8.68 and 12.31, respectively, by spectrophotometry. The enthalpies of the complexation (Delta H-1 and Delta H-2) were measured to be -25.2 and -45.9 kJ/mol by microcalorimetry. The entropies (Delta S-1, and Delta S-2) were calculated to be 81.6 and 81.8 J/(K mol) accordingly. The strong complexation of NpO2+ with DPA is driven by both positive entropies and highly exothermic enthalpies. The crystal structure of Na3NpO2(DPA)(2)(H2O)(6)(S) shows that, in the NpO2-(DPA)(2)(3-) complex, the Np atom sits at a center of inversion and the two DPA ligands symmetrically coordinate to Np in a tridentate mode. Due to the centrosymmetric structure of the NpO2(DPA)(2)(3-) complex, the f -> f transitions of Np(V) are forbidden, and the sharp bands originating from the f -> f transitions either disappear or become very weak in the optical absorption or diffuse reflectance spectra of the NpO2(DPA)(2)(3-) complex.