A study of neptunium (Np) chemistry in the complex oxo-selenium system has been performed. Hereby, two sets of precipitation experiments were conducted, investigating the influence of the initial oxidation state of selenium using (SeO2)-O-IV and (H2SeO4)-O-VI with Np-V in alkali nitrate solution, keeping the ratio of Np/Se constant. Surprising results were observed. Five novel neptunium and selenium bearing compounds have been obtained by slow evaporation from aqueous solution. The novel Np-IV phase K4-x[Np(SeO3)(4-x)(HSeO3)x]center dot(H2O)(1.5) (1) crystallizes in green-colored, plate-shaped crystals and was obtained by adding SeO2 and ANO(3) to a Np-v stock solution. Single-crystal X-ray diffraction reveals one-dimensional chain structures composed of square antiprismatic NpO8 polyhedra linked via four trigonal pyramidal SeO3 and HSeO3 units. Raman spectral analysis supports the presence of both selenite and hydroselenite due to the presence of corresponding modes within the spectra. The addition of selenic acid to a Np-V stock solution resulted in the precipitation of elongated rose prisms of K-2[(NpO2)(2)(SeO4)(3)(H2O)(2)]center dot(H2O)(1.5) (2), Rb-2[(NpO2)(2)(SeO4)(3)(H2O)(2)]center dot(H2O)(2) (3) and K-9[(NpO2)(9)(SeO4)(13.5)(H2O)(6)]center dot (H2O)(12) (4) as well as light red plates of Cs-2[(NpO2)(2)(SeO4)(3)] (5). To our knowledge, this is the first report of Npvl selenates. All four structures show two-dimensional layered structures with alkali cations acting as charge balancing counter cations. Hereby the layers of compounds 2 and 3 are found to be orientational geometric isomers. Distinctly different phenomena are made responsible for the phase formation within these systems. The kinetically driven process of Np-v disproportionation led to the formation of the Np-IV selenites in the Sew-based system, whereas the oxidation of Np-v by reduction of nitrate in acidic conditions is responsible for the formation of the Np-VI selenates in the Se-VI system. The influence of air oxygen is also discussed for the latter reaction.