The crystal structure of ammonium μ-fluoro-μ-oxo-bis [oxodiperoxomolybdate(VI)]-water(2/1), (NH 4 ) 3 [FO{MoO(O 2 ) 2 } 2 H 2 O, has been determined from single-crystal X-ray diffraction data. The yellow compound crystallizes as multiple-twins in the monoclinic space group P2 1 /c with a = 13.803(3) a, b = 11.230(2) a, c = 15.975(3) a, β = 109.46(2) o and Z = 8. Full-matrix least-squares refinement of 316 structural parameters gave R = 0.057 for 3265 observed [I>3σ(I)], independent reflections. The [FO{MoO(O 2 ) 2 } 2 ] 3 - anions, of which there are two crystallographically different ones in the asymmetric unit, have approximately C 2 v symmetry and are composed of two edge-sharing pentagonal bipyramids, the molybdenum atoms thus being seven-coordinated. The molybdenum atoms are displaced 0.312-0.365 a from the equatorial planes towards the respective double-bonded apical oxygen atom. Bond distances are as follows: Mo-O p e r o x o 1.884(10)-1.975(8) a, Mo = O a p i c a l 1.670(8)- 1.682(8)am Mo-F b r i d g e 2.246(6)-2.313(6)a, Mo-O b r i d g e 1.925(7)-1.941(7) a and (O-O) p e r o x o 1.436(15)-1.465(12) a. Mo...Mo is 3.369(1)-3.379(1) a, Mo-O-Mo 120.5(4)-122.6(4) o and Mo-F-Mo 94.0(2)-95.5(2) o . The powder photograph of the title compound was recorded at room temperature and was indexed.