A hard X-ray photoelectron spectroscopy (HX-PES) has been employed in order to investigate electrochemical polarization at carbon/amorphous tantalum oxide thin film hetero-interface. In situ HX-PES observation of Ta 4f and O 1s spectra has revealed parallel chemical shift of the core levels within the redox window width of 2.4 eV, indicating Fermi level shifts due to the redox state modulation mediated by protonation and deprotonation under applied dc bias voltage. In addition to protons, oxide ions are suggested to take part in the redox state variation especially under a larger applied electric field. The results have been discussed in framework of solid state electrochemistry and Pourbaix-type E-pO diagram to indicate electrochemical stability domains of solid oxide systems is proposed. (C) 2013 Elsevier B.V. All rights reserved.