Heacavalent neptunium can be solubilized in 0.5-3.5 M aqueous MOH (M = Li+, Na+, NMe4+ = TMA(+)) solutions. Single crystals were obtained from cooling of a dilute solution of Co(NH3)(6)Cl-3 and NpO22+ in 3.5 M [N(Me)(4)]OH to 5 C. A single-crystal X-ray diffraction study revealed the molecular formula of [Co(NH3)(6)](2)[NpO2(OH)(4)](3)center dot H2O, isostructural with the uranium analogue. The asymmetric unit contains three distinct NpO2(OH)(4)(2-) ions, each with pseudooctahedral coordination geometry with trans-oxo ligands. The average Np=O and Np-OH distances were determined to be 1.80(1) and 2.24(1) angstrom, respectively. EXAFS data and fits at the Np L-III-edge on solid [Co(NH3)(6)](2)[NpO2(OH)(4)](3)center dot H2O and aqueous solutions of NpO22+ in 2.5 and 3.5 M (TMA)OH revealed bond lengths nearly identical with those determined by X-ray diffraction but with an increase in the number of equatorial ligands with increasing (TMA)OH concentration. Raman spectra of single crystals of [Co(NH3)(6)](2)[NpO2(OH)(4)](3)center dot H2O reveal a nu(1)(O=Np=O) symmetric stretch at 741 cm(-1). Raman spectra of NpO22+ recorded in a 0.6-2.2 M LiOH solution reveal a single nu(1) frequency of 769 cm(-1). Facile exchange of the neptunyl oxo ligands with the water solvent was also observed with Raman spectroscopy performed with O-16- and O-18-enriched water solvent. The combination of EXAFS and Raman data suggests that NpO2(OH)(4)(2-) is the dominant solution species under the conditions of study and that a small amount of a second species, NpO2(OH)(5)(3-), may also be present at higher alkalinity. Crystal data for [Co(NH3)(6)](2)[NpO2(OH)(4)](3)center dot H2O: monoclinic, space group C2/c, a = 17.344(4) angstrom, b = 12.177(3) angstrom, c = 15.273 angstrom, beta = 120.17(2)degrees, Z = 4, R1 = 0.0359, wR2 = 0.0729.