Negatively charged 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF(4)) | Mo2C derived micromesoporous carbon (C) electrode interface has been studied using in situ synchrotron radiation initiated X-ray photoelectron spectroscopy (XPS) in order to estimate true potentials when slow electrochemical processes start at the C electrode used in the supercapacitors. A three-electrode system within the very wide working electrode (WE) potential region down to -1.90 V (vs. Ag/AgCl wire in EMImBF(4)) has been used for the polarization of the C WE during the in situ XPS measurements. Parallel to XPS measurements in high vacuum chamber, the cyclic voltammetry measurements in the three-electrode system were carried out in the Ar-filled glovebox in order to obtain electrochemical data down to -2.40 V (vs. Ag/AgCl wire in EMImBF(4)) for studied C electrode material. WE potential vs. XPS spectra relationship has been established and discussed here. (c) 2015 Elsevier Ltd. All rights reserved.