A study of the fused salt system Ag 2MoO 4MoO 3 by X-ray diffraction techniques has shown the existence of two new phases, silver dimolybdate, Ag 2Mo 2O 7, and silver decamolybdate, Ag 6Mo 10O 33. Silver decamolybdate is triclinic, space group P overline1 with lattice parameters a = 7.59, b = 8.31, c = 11.42 Å, α = 82.6°, β = 102.9° and γ = 106.4°; there is one formula weight in the unit cell. Its structure, determined by three-dimensional Patterson and Fourier methods and refined by least-squares techniques, contains distorted MoO 6 octahedra grouped by edge-sharing into sub units of four and six; these units then link up by common corners and edges to form infinite sheets. Some of the silver ions occupy irregular five-coordinate sites within the sheets, whilst the remainder occupy irregular seven-coordinate interlayer positions. Adjacent layers have no common oxygen atoms. Similarities to the red and blue potassium molybdenum bronzes, and to the hepta- and octa-molybdate polyanions are discussed.