Stability of different phases of AMoO 4 (A = Mg, Ni) molybdates versus A–O bonding and the corresponding electronic structures are examined from first principles. The energy-volume equations of state for three forms (β, α, ω), characterized by decreasing volumes in the sequence of Mg and Ni molybdates are established. While NiMoO 4 is energy stabilized in the sequence β → α → ω, an opposite behavior is identified for the Mg molybdate. Charge analysis characterizing ionic Mg 2+ versus covalent Ni +1.2 behaviors can explain the trend. Electronic band structure also shows large differences: MgMoO 4 is insulating with a ∼2 eV band gap while in a magnetic state, NiMoO 4 is a small gap (∼0.2 eV) semi-conductor. Chemical bonding properties show weak Mg and strong Ni bonding with oxygen, while identifying the Mo–O interaction as prevailing.