A magnesium deuteride preparation of composition Mg(D0.gH0.1) 2 is found to be tetragonal with a=4.5025, c=3.0123/~. The atomic positions are:2 Mg in (000) ~922/ , 1~1-~ 4(D +H) in _+ (xxO) (½ +x, t-x, 1) with x =0.306 _+0"003.The bond lengths are Mg-6D = 1.95 + 0.02 A. The structure is of the rutile type, Some years ago it was shown by X-ray diffraction (Ellinger et al., 1955) that magnesium hydride is tetragonal with two molecules in a unit cell of dimensions a--4.5618_+ 0.0005, c=3.0205_+ 0.0005 A.The two magnesium atoms were found to be in positions (0, 0, 0)( I, ½, I). It was suggested that the hydrogen atoms were located at the sites +_(x,x, 0)G+x, ½-x, I) * Work done under the auspices of the U.S. Atomic Energy Commission. with x ~ 0.306, i.e. that MgHe has the rutile type of structure. However, the hydrogen atoms had no measurable effect on the diffraction intensities, and accordingly there was no experimental proof for the proposed hydrogen positions. A direct determination of the hydrogen sites by means of neutron diffraction was therefore undertaken, using the deuteride in preference to the hydride.Magnesium deuteride was prepared by reaction of deuterium gas with magnesium metal (obtained by decomposition of MgH2) for four days at 510 °C.