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Through the use of ambient pressure X-ray photoelectron spectroscopy (APXPS) and a single-sided solid oxide electrochemical cell (SOC), we have studied the mechanism of electrocatalytic splitting of water (H2O + 2e- → H2 + O2-) and electro-oxidation of hydrogen (H2 + O2- → H2O + 2e-) at 700 °C in 0.5 Torr of H2/H2O on ceria (CeO2-x) electrodes. The experiments reveal a transient build-up of surface intermediates (OH- and Ce3+) and show the separation of charge at the gas-solid interface exclusively in the electrochemically active region of the SOC. During water electrolysis on ceria, the increase in surface potentials of the adsorbed OH- and incorporated O2- differ by 0.25 eV in the active regions. For hydrogen electro-oxidation on ceria, the surface concentrations of OH- and O2- shift significantly from their equilibrium values. These data suggest that the same charge transfer step (H2O + Ce3+ ⇔ Ce4+ + OH- + H•) is rate limiting in both the forward (water electrolysis) and reverse (H2 electro-oxidation) reactions. This separation of potentials reflects an induced surface dipole layer on the ceria surface and represents the effective electrochemical double layer at a gas-solid interface. The in situ XPS data and DFT calculations show that the chemical origin of the OH-/O2- potential separation resides in the reduced polarization of the Ce-OH bond due to the increase of Ce3+ on the electrode surface. These results provide a graphical illustration of the electrochemically driven surface charge transfer processes under relevant and nonultrahigh vacuum conditions. [ABSTRACT FROM AUTHOR]