Mg6Co2D11 crystallizes in space group Pnma, a=8.1000(2)Å, b=10.0643(2)Å, c = 18.5664(4)Å, V = 1513Å3, Z = 8. The partly disordered structure contains 14 symmetry-independent deuterium atoms, of which 9 are coordinated to cobalt in square-pyramidal [CoD5]4− and saddle-like [CoD4]5− complex anions, and 5 are bonded to magnesium only. The Co-D bond distances range between 1.52(1) Å and 1.62(2) Å. The limiting ionic formula can be written as 4Mg6Co2D11=5MgD219Mg2+2[CoD5]4−6[CoD4]5−. The 63-parameter structure was solved ab initio and refined from joint synchrotron and neutron high resolution powder diffraction data.