A mechanism of water oxidation catalyzed by the carbon-free tetra-Co containing polyoxometalates [Co₄(H₂O)₂(PW₉O₃₄)₂]¹⁰⁻ (PCo₄) and [Co₄(H₂O)₂(VW₉O₃₄)₂]¹⁰⁻(VCo₄) is elucidated by DFT calculations. Computational analysis for PCo₄ suggests that a first PCET step may proceed via a sequential electron-then-proton transfer (ET + PT) pathway and leads to one electron oxidize species S₁ (POM-Co^IIIOH). In contrast, the second PCET, which controls the potential required to form POM-Co^IIIOactivespecies S₂ is clearly a concerted process. The overall S₀ → S₂ transformation is estimated to require less than 1.48 V and 1.62 V applied potential at pH = 8 for PCo₄ and VCo₄ anions, respectively. At operando conditions, with the presence of a buffer and with an applied potential above the threshold potential the two H-atom removal could take place via concerted pathways. These steps represent rapid pre-equilibria before the rate determining step, which corresponds to the OO bond formation. The key chemicalstep occurs via nucleophilic attack of an external water molecule to intermediate S₂. We assume that this step governs the kinetics of the reaction. Comparison of the calculated energetics and electronic structures of intermediate species in the PCo₄ and VCo₄catalyzed water oxidation cycle shows that coupling of d orbitals of V and Co atoms in VCo₄ increases the oxidation potential of the Co-center. The orbital coupling is also responsible for the higher catalytic activity of VCo₄ because it increases the electrophilicity of Co^IIIO moiety in the key S₂ species.