For H 2 O reduction on CeO 2 , the gradient ∂∆χ( ⃗ E)/∂η act ( ⃗ E) = 0.65 is in good agreement with the linear fit to the experimental data ∂∆χ/∂η act = 0.76 (Fig. ). The error can be attributed to lateral interactions between dipoles which occurs when the adsorbate coverage is greater than approximately 1%, where the intrinsic dipole moment imposed by the adsorbate is dependant on the coverage. DFT calculations were carried out at θ OH • O = 0.125 which is close to the experimentally measured coverage at PZC, and where the model and data best agree. However, as η act is increased the model and experimental results deviate as the hydroxyl coverage decreases, increasing the strength of the intrinsic dipole moment and increasing the gradient ∂∆χ/∂η act . Mechanistically, the ambipolar charge transfer reaction is decoupled, where the fast adsorption step (Table ) fills a charged oxygen vacancy (IT), while the ET step becomes rate limiting. Table illustrates the ∆χ -η act relationships derived from each reaction step, where we have shown that ET is rate limiting to derive the experimental trend ∆χ ≈ η act .