Mg batteries utilizing oxide cathodes can theoretically surpass the energy density of current Li-ion technologies. The absence of functional devices so far has been ascribed to impeded Mg2+ migration within oxides, which severely handi-caps intercalation reactions at the cathode. Broadly, knowledge of divalent cation migration in solid frameworks is surprisingly deficient. Here we present a combined experimental and theoretical study of Mg migration within three spinel oxides, which reveal critical features that influence it. Experimental activation energies for a Mg22+ hop to an adjacent vacancy, as low as ~600 meV, are reported. These barriers are low enough to support functional electrodes based on the intercalation of Mg2+. Subsequent electrochemical experiments demonstrate that significant demagne-siation is indeed possible, but that challenges instead lie with the chemical stability of the oxidized states. Our find-ings enhance the understanding of cation transport in solid structures and renew the prospects of finding materials capable of high density of energy storage.