The hydrothermal reaction of NpO2 with IO3- in the presence of nitrate results in the formation of NpO2(IO3) (1). Under similar conditions, NpO2 reacts with AgNO3 and SeO2 to yield alpha-AgNpO2(SeO3) (2) and beta-AgNpO2(SeO3) (3). The structure of 1 consists of distorted pentagonal bipyramidal Np(V) centers that are bridged by iodate anions. In addition, the oxo atoms of the neptunyl(V) cations coordinate adjacent Np(V) centers creating layers that are linked into a three-dimensional network structure by the iodate anions. The structure is polar owing to the alignment of the stereochemically active lone pair of electrons on the iodate anions along the c-axis. alpha-AgNpO2(SeO3) (2) forms a layered structure consisting of hexagonal bipyramidal NpO8 polyhedra that are bound by chelating and bridging selenite anions. The primary and secondary structures of 3 are similar to those of 1, and neptunyl-neptunyl interactions are partially responsible for the creation of a three-dimensional network structure. However, the selenite anions in 3 are rotated with respect to the iodate anions found in 1, and the structure is centrosymmetric. The network found in 3 consists of interconnecting, approximately square channels that house the Ag+ cations. A bond-valance sum parameter of 2.036 Angstrom for Np(V) bound exclusively to oxygen has been developed with b = 0.37 Angstrom. Crystallographic data: 1, orthorhombic, space group Pna2(1), a = 13.816(2) Angstrom, b = 5.8949(8) Angstrom, c 5.5852(8) Angstrom, Z = 4; 2, monoclinic, space group P2(1)/n(1) a = 4.3007(3) Angstrom, b = 9.5003(7) Angstrom, c = 11.5877(g) Angstrom, beta = 95.855(1)degrees, Z = 4; 3, triclinic, space group P1, a = 7.1066(6) Angstrom, b = 8.3503(7) Angstrom, c = 8.3554(7) Angstrom, alpha = 89.349(1)degrees, beta = 77.034(1)degrees, gamma = 76.561 (1)degrees, Z = 2.