Abstract: A molecular-level understanding of the electrical double layer (EDL) on graphene is critical for the electrochemical energy storage of carbon-based electrodes. In this work, the electrochemical interface between single-layer graphene (SLG) and an ionic liquid (IL, EMI⁺TFSI⁻) electrolyte is investigated by using cyclic voltammetry, electrochemical impedance spectroscopy, in situ Raman spectroscopy and in situ attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy. In the charge/discharge voltage range from −1.0 V to 1.0 V, the SLG is electrochemically doped due to the interaction between adsorbed ions and SLG. For a voltage larger than 1.75 V or lower than −2.0 V, the irreversible formation of structural defects is detected on SLG, attributed to the decomposition of EMI⁺TFSI⁻ and the sequential reaction. In situ ATR-FTIR suggests a potential-dependent reorientation of ions: the imidazolium ring of EMI⁺ is tilted at low negative and positive polarization and then lifts away from the SLG surface at a higher positive potential (> 0.6 V), and the rearrangement of TFSI⁻ causes an increased adsorption density at positive potentials. Our findings provide deeper insight into the EDL structure on graphene down to the molecular level and may impact the design of carbon supercapacitors with higher energy storage capacity.