The formation process from the intermetallic compound LaMg2Ni to a complex hydride (deuteride) LaMg2NiD7 composed of Le(3+), 2 x Mg2+, [NiD4](4-), and 3 x D- was investigated by in -situ powder neutron diffraction under deuterium gas pressure at room temperature. Below 0.001 MPa, small amount of deuterium was initially dissolved in the lattice of LaMg2Ni forming LaMg2NiD0.05 and two new hydride phases (LaMg2NiDx1 and LaMg2NiDx2) were continuously yielded. Furthermore, LaMg2NiD4.6 with NiD1.6 and NiD3.3 units and interstitial deuterium atoms was formed prior to appearing of LaMg2NiD7. From their Bragg peak positions, the deuterium contents x1, and x2 were inferred as 0.05 < x1 < x2 < 4.6. At approximately 0.001 MPa, LaMg2NiD7 started forming. Since the intermediate deuterides had similar metal atomic framework of LaMg2Ni, we revealed that LaMg2NiD7 formation proceeded through multiple deuteride phases with maintaining of the metal atomic framework of LaMg2Ni. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.