While Mg-Mg 2 Ni composites are promising for hydrogen storage, their implementation is hindered by our incomplete understanding of absorption/desorption kinetics. Here, we combine in situ neutron diffraction with kinetic and microstructural analyses to uncover the sorption mechanism of deuterated hydrogen D 2 in a Mg-Mg 2 Ni composite processed by fast forging. Phase transitions upon first absorption are found to be different from subsequent absorptions. The first absorption involves rapid formation of Mg 2 NiD 0.3-x followed by simultaneous formation of MgD 2 and Mg 2 NiD 4 . Kinetic modeling indicates that surface nucleation of the magnesium hydride is rate-limiting. Subsequent absorptions involve two phases, Mg and Mg 2 NiD 0.3-x , which promote absorption. Kinetic modeling and microstructure analysis indicate that (1) MgD 2 nucleation occurs at the Mg-Mg 2 NiD 0.3-x interface and (2) Mg 2 NiD 4 formation is kinetically controlled by deuterium diffusion through the growing Mg 2 NiD 4 plate. In all desorptions, deuterium release starts by rapid decomposition of Mg 2 NiD 4 into Mg 2 NiD 0.3-x , followed by slower MgD 2 decomposition.