The hydrothermal syntheses, single-crystal structures, and some properties of two new layered molybdenum selenites, (NH4)(2)(MoO3)(3)SeO3 and Cs-2(MoO3)(3)SeO3, are described. These phases are built up from infinite, hexagonal tungsten bronze-like, anionic layers of MoO6 octahedra, capped on one side by pyramidally-coordinated Se atoms. The molybdenum-oxygen octahedra show an unusual distortion of the Mo atom towards an octahedral face, resulting in a 3 short +3 long Mo-O bond distance distribution within the MoO6 unit. Charge compensation is provided by interlayer NH4+ or Cs+ cations. Powder X-ray, infrared, Raman and thermogravimetric data for these phases are also presented and discussed. The M(2)(MoO3)(3)SeO3 (M = NH4, Cs) structure is related to, but distinctly different from the structure of NH4(VO2)(3)(SeO3)(2) recently reported. Crystal data : (NH4)(2)(MoO3)(3)-SeO3, M(r) = 594.85, hexagonal, space group P6(3) (No, 173), a = 7.267(2) Angstrom, c = 12.031(3) Angstrom, V = 550.3(3) Angstrom(3), Z = 2, R = 3.26%, R(w) = 5.61% [559 observed reflections with I > 3 sigma(I)]; Cs-2(MoO3)(3)SeO3, M(r) hexagonal, space group P6(3) (No. 173), a = 7.312(2) Angstrom, c = 12.377(2) Angstrom, V = 573.1(3) Angstrom 3, Z = 2, R = 5.93%, R(w) = 6.52% [514 observed reflections with I > 3 sigma(I)].