The crystal structure of (NH4)6Mo8O27.4H2O has been determined from three‐dimensional X‐ray diffractometer data and refined to anRindex of 6.4%. The compound was obtained from a concentrated aqueous ammonium heptamolybdate solution after several weeks. It crystallizes in the space groupP21/cwitha= 9519,b= 11,309,c= 15.049 Å, β = 109.50°, andZ= 2. The structure consists of octamolybdate units which are polymerized to infinite chains through one common oxygen atom. The octamolybdate unit is built up of distorted MoO6octahedra sharing edges and corners. Mo‐O bond lengths vary from 1.697 to 2.401 Å, with values of 1.697 to 1.732 Å in the terminal bonds. Mo … Mo distances are 3.258 to 3.472 Å (edge‐sharing), 3.778, 3.970 and 3.980 Å (corner‐sharing). The anion chains are separated by NH4+ions and H2O molecules; a probable network of hydrogen