Electrochemical processes at negatively polarized electrical double layer capacitor (EDLC) electrode at different cell potentials have been studied using in situ synchrotron radiation excited X-ray photoelectron spectroscopy (XPS). 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMImBF(4)) room-temperature ionic liquid (RTIL), as an electrolyte, and Mo2C derived carbon (C(Mo2C)) based micromesoporous electrodes, as an EDLC electrodes, were studied within very wide cell potential region (up to 4.0 V). To store more charge in a RTIL based EDLC, higher cell potentials have been applied leading to the cross-over of the limit of the ideal polarization of the capacitor electrodes and to the initiation of different faradaic processes. Therefore, parallel to XPS measurements, the cyclic voltammetry was used to obtain electrochemical data, correlated with previously calculated electrochemical (including electrochemical impedance spectroscopic) characteristics for C(Mo2C) vertical bar RTIL interface. In this paper we have, according to our knowledge, first polarized the RTIL based EDLC electrodes in real in situ vacuum conditions, measured and analyzed the supercapacitor two-electrode cell potential vs. XPS spectra relationship and discussed in the light of XPS data possible electrochemical reactions taking place at the negatively charged working electrode vertical bar RTIL interface at the different cell potential applied. (C) 2013 The Electrochemical Society. All rights reserved.