Green crystals of (NpO2)(2)(SeO4)(H2O)(4), (NpO2)(2)(SeO4)(H2O)(2), and (NpO2)(2)(SeO4)(H2O) have been prepared by hydrothermal methods. The structures of these compounds have been characterized by single-crystal X-ray diffraction; (NpO2)(2)(SeO4)(H2O)(4), isostructural with (NpO2)(2)(SO4)(H2O)(4), is constructed from layers comprised of corner-sharing neptunyl(V) pentagonal bipyramids and selenate tetrahedra that are further linked by hydrogen bonding with water molecules. Each NpO2+ cation binds to four other NpO2+ units through cation cation interactions (CCIs) to form a distorted "cationic square net" decorated by SeO42- tetrahedra above and below the layer. Each selenate anion is bound to two neptunyl(V) cations through monodentate linkages. (NpO2)(2)(SeO4) (H2O)(2) is isostructural with the corresponding sulfate analogue as well. It consists of puckered layers of neptunyl (V) pentagonal bipyramids that are further connected by selenate tetrahedra to form a three-dimensional framework. The CCI pattern in the neptunyl layers of dihydrate is very similar to that of tetrahydrate; however, each SeO42-. tetrahedron is bound to four NpO2+ cations in a mondentate manner. (NpO2)(2)(SeO4) (H2O) crystallizes in the monoclinic space group P2(1)/c, which differs from the (NpO2)(2)(SO4)(H2O) orthorhombic structure due to the slightly different connectivities between NpO2+ cations and anionic ligands. The structure of (NpO2)(2)(SeO4)(H2O) adopts a three-dimensional network of distort neptunyl(V) pentagonal bipyrarnids decorated by selenate tetrahedra. Each NpO2+ cation connects to four other NpO2+ units through CCIs and also shares an equatorial coordinating oxygen atom with one of the other units in addition to the CC bond to form a dimer. Each SeO42- tetrahedron is bound to five NpO2+ cations in a monodentate manner. Magnetic measurements obtained from the powdered tetrahydrate are consistent with a ferromagnetic ordering of the neptunyl(V) spins at 8(1) K, with an average low temperature saturation moment of 1.98(8) mu(B) per Np. Well above the ordering temperature, the susceptibility follows Curie-Weiss behavior, with an average effective moment of 3.4(2)mu(B) per Np and a Weiss constant of 14(4) K Correlations between lattice dimensionality and magnetic behavior are discussed.