The structure of Mg₆C0₂H₁₁ was solved ab initio from five powder diffraction data sets: Synchrotron data, λ=1.6095, 1.6195 and 1.6295 Å, neutron data, λ=1.7070 Å and neutron data λ=1.5939 Å. The average structure of the metal atoms was solved in space group Cmcm by Patterson and Fourier synthesis. The superstructure with four times greater cell was solved in space group Pnma (cell parameters of deuteride: a = 8.1000(2) Å, b = 10.0643(2) Å, c = 18.5664( 4) Å, Z = 8). The occupancies of the possible deuterium sites were found from the neutron data by Rietveld refinement. The final refinement was done simultaneously on 3 data sets. The joint refinement of 102 parameters, with 63 positional and 13 isotropic temperature parameters converged to the following values of agreement indices: R_p = 12 %, R_B = 9 %, S = 1.01.