The compound NH4(VO2)(3)(SeO3)(2) has been prepared by hydrothermal synthesis at 200 degrees C from V2O5, SeO2, and NH4Cl. The crystal structure was determined by single-crystal X-ray diffraction. Crystal data : NH4(VO2)(3)(SeO3)(2), M(r) = 410.79; hexagonal, space group P6(3) (No. 173); cell parameters a = 7.137(3) Angstrom A, c = 11.462(4) degrees A, V = 505.7(4) Angstrom A(3), Z = 2; R(F) = 0.0416 and R(w)(F-2) = 0.1041 (515 reflections). The structure consists of hexagonal layers of corner-sharing VO6 octahedra. The apical oxygen atoms of the VO6 octahedral are capped by Se atoms that form SeO32- groups; the ammonium ions are located between adjacent layers. The structure contains the first example of a hexagonal tungsten bronze type layer where the octahedral cation is vanadium. Redox intercalation reactions similar to those reported for VOPO4 H2O were studied but are complicated by the ease of reduction of the SeO32- group to selenium metal accompanied by decomposition of the vanadium-oxygen layers acid the formation of V2O5.