The dehydriding of bulk Mg2NiH4 is shown in real-time using ultra-high voltage TEM. The dehydriding occurs via a mechanism of nucleation and growth of Mg2NiHx (x∼0–0.3). In-situ Synchrotron XRPD and DSC further support the observed behaviour of Mg2NiH4. A high density of stacking faults is observed in the dehydrided lattices by ARM-TEM.