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Abstract: Magnesium nickel hydrides (Mg2NiH4) are the prospective candidates for hydrogen storage and switchable mirror. The hydrides exist in two typical crystallographic forms, the low temperature (LT) phase in monoclinic structure, and the high temperature (HT) phase in cubic structure. LT has two modifications–untwinned (LT1) and microtwinned (LT2) structures. The electronic structures of the three polymorphs of Mg2NiH4 are investigated using ab initio calculations based on density functional theory. The calculated band gaps of LT1 and HT are in reasonable agreement with experimental observations and other theoretical predications, while the calculated band gap of LT2 is slightly lower than those of LT1 and HT. Electronic-structure analysis shows that strong interactions exist between Ni and H, whereas the interactions between Mg and H are negligible. The strong ionic character between Mg and NiH4 complex can be viewed as the origin of the semiconducting ground-state. [Copyright &y& Elsevier]