Hydrated neptunium-(IV),-(V), and-(VI) nitrates, Np(NO3)4,2H2O (grey), NpO2NO3,H2O (green), NpO(NO3)3,3H2O (pink), and NpO2(NO3)2,6H2O (pink), have been prepared from aqueous nitric acid. Solvated neptunium-(IV) and -(VI) nitrates, Np(NO3)4,xN2O5(x∼ 1·2)(off-white) and NpO2(NO3)2,N2O5,H2O (pale pink), have been prepared by reaction of neptunium tetrachloride and neptunium trioxide hydrate, respectively, with dinitrogen pentoxide. The neptunium tetranitrate dimethylacetamide (DMA) complex, Np(NO3)4,2·5DMA (grey-green), has been prepared by reaction of the dihydrate with the ligand, or from the corresponding chloride complex and silver nitrate in methyl cyanide. The attempted preparation of neptunium(V) nitrato- and oxynitrato-complexes was unsuccessful. Magnetic properties, infrared spectra, and some X-ray diffraction measurements are also presented.