A systematic investigation is presented which examines the structural chemistry of anhydrous and hydrous ternary hexavalent uranium and neptunium dinitrato, trinitrato, and tetranitrato complexes. Using slow evaporation methods under acidic conditions the uranium and neptunium nitrate complexes gamma-K[UO2(NO3)(3)], K-2[UO2-cis-(NO3)(4)], [NpO2(NO3)(2)(H2O)(2)]center dot 4H(2)O, and Cs-[NpO2(NO3)(3)] have been synthesized and their structures refined using single-crystal X-ray diffraction data. gamma-K[UO2(NO3)(3)] adopts an orthorhombic structure in space group Pbca consisting of antiparallel aligned [UO2(NO3)(3)](-) moieties. K-2[UO2-cis-(NO3)(4)] adopts a monoclinic structure in space group P2(1)/c consisting of [UO2(NO3)(4)](2-) moieties with two monodentate and two bidentate nitrate ligands that are arranged in a cis configuration about the uranyl, UO22+, center. Previous investigations have only identified trans variants of this monoclinic structure, and this is the first report of the cis form and also the occurrence of geometric isomerism in uranyl nitrates. [NpO2(NO3)(2)(H2O)(2)]center dot 4H(2)O adopts an orthorhombic structure in space group Cmc2(1) consisting of parallel aligned [NpO2(NO3)(2)(H2O)(2)] moieties that are in a trans configuration with respect to the bidentate nitrate ligands. Cs[NpO2(NO3)(3)] adopts a hexagonal structure in space group R (3) over barc consisting of parallel aligned [NpO2(NO3)(3)](-) moieties. It was found that despite using a Np(V) nitrate solution as the starting reagent, Np(VI) nitrate structures were consistently recovered under acidic conditions. These observations are discussed and rationalized with respect to standard reduction potentials, particularly how redox conditions and acidity affect the oxidation state of Np and subsequent structure formation. The structures uncovered in this investigation are discussed comparatively and systematically in detail with other reported anhydrous and hydrous ternary hexavalent uranium and neptunium dinitrato, trinitrato, and tetranitrato complexes, particularly with respect to how synthesis conditions, including pH and geometric isomerism, affect the structural chemistry.